diff --git a/app/models/concerns/triggerable_hooks.rb b/app/models/concerns/triggerable_hooks.rb
index 64e4a3311dc91c7689da20ab436273d30dafcc95..a892593946b640ee6373842e75c47ad16ffc3163 100644
--- a/app/models/concerns/triggerable_hooks.rb
+++ b/app/models/concerns/triggerable_hooks.rb
@@ -32,6 +32,7 @@ def available_triggers
         job_hooks: :job_events,
         member_hooks: :member_events,
         merge_request_hooks: :merge_requests_events,
+        milestone_hooks: :milestone_events,
         note_hooks: :note_events,
         pipeline_hooks: :pipeline_events,
         project_hooks: :project_events,
diff --git a/app/models/hooks/project_hook.rb b/app/models/hooks/project_hook.rb
index 1226a76433bfafd9cd8a51f1733589f27002fd62..c15482d071eaae339b4d1f00d863f06450f80aec 100644
--- a/app/models/hooks/project_hook.rb
+++ b/app/models/hooks/project_hook.rb
@@ -15,6 +15,7 @@ class ProjectHook < WebHook
     :issue_hooks,
     :job_hooks,
     :merge_request_hooks,
+    :milestone_hooks,
     :note_hooks,
     :pipeline_hooks,
     :push_hooks,
diff --git a/app/models/milestone.rb b/app/models/milestone.rb
index 358b479476489fa4d100e2d5205c5f695370de4c..7bdbdf49f7c59a0b4bfa7f32527e7663e4314de0 100644
--- a/app/models/milestone.rb
+++ b/app/models/milestone.rb
@@ -99,6 +99,10 @@ class Predefined
     state :active
   end
 
+  def hook_attrs
+    Gitlab::HookData::MilestoneBuilder.new(self).build
+  end
+
   # Searches for timeboxes with a matching title.
   #
   # This method uses ILIKE on PostgreSQL
diff --git a/app/services/concerns/integrations/project_test_data.rb b/app/services/concerns/integrations/project_test_data.rb
index 185b57a1384555d0d303fd7c2ed64a9596e57d02..f71cf87832ca43cabb72b9a9da6d4eab8842eb64 100644
--- a/app/services/concerns/integrations/project_test_data.rb
+++ b/app/services/concerns/integrations/project_test_data.rb
@@ -82,6 +82,10 @@ def releases_events_data
       release.to_hook_data('create')
     end
 
+    def milestone_events_data
+      Gitlab::DataBuilder::Milestone.build_sample(project)
+    end
+
     def emoji_events_data
       no_data_error(s_('TestHooks|Ensure the project has notes.')) unless project.notes.any?
 
diff --git a/app/services/integrations/test/project_service.rb b/app/services/integrations/test/project_service.rb
index 1e077d49e5a0cefaf5476d4c3c223f3d95b81788..3334b241dddae61f5446d0641ae937e56b517b60 100644
--- a/app/services/integrations/test/project_service.rb
+++ b/app/services/integrations/test/project_service.rb
@@ -14,7 +14,7 @@ def project
 
       private
 
-      def data
+      def data # rubocop:disable Metrics/CyclomaticComplexity -- despite a high count of cases, this isn't that complex
         strong_memoize(:data) do
           case event || integration.default_test_event
           when 'push', 'tag_push'
@@ -35,6 +35,8 @@ def data
             deployment_events_data
           when 'release'
             releases_events_data
+          when 'milestone'
+            milestone_events_data
           when 'award_emoji'
             emoji_events_data
           when 'current_user'
diff --git a/app/services/milestones/base_service.rb b/app/services/milestones/base_service.rb
index 0d7d855bf5ed3bf3f73b0b831b909514ed45bca9..ecfe7db325e51436c3c0b570de8f195ac40a4155 100644
--- a/app/services/milestones/base_service.rb
+++ b/app/services/milestones/base_service.rb
@@ -2,14 +2,26 @@
 
 module Milestones
   class BaseService < ::BaseService
-    # Parent can either a group or a project
     attr_accessor :parent, :current_user, :params
 
     def initialize(parent, user, params = {})
+      # Parent can either a group or a project
       @parent = parent
       @current_user = user
       @params = params.dup
+
       super
     end
+
+    private
+
+    def execute_hooks(milestone, action)
+      # At the moment, only project milestones support webhooks, not group milestones
+      return unless milestone.project_milestone?
+      return unless milestone.parent.has_active_hooks?(:milestone_hooks)
+
+      payload = Gitlab::DataBuilder::Milestone.build(milestone, action)
+      milestone.parent.execute_hooks(payload, :milestone_hooks)
+    end
   end
 end
diff --git a/app/services/milestones/close_service.rb b/app/services/milestones/close_service.rb
index a252f5c144ea142896ea1de1aeda811e8c78644b..616c6d85b7c55a1cbebaa7ec5cb423933a02e5f7 100644
--- a/app/services/milestones/close_service.rb
+++ b/app/services/milestones/close_service.rb
@@ -5,6 +5,7 @@ class CloseService < Milestones::BaseService
     def execute(milestone)
       if milestone.close && milestone.project_milestone?
         event_service.close_milestone(milestone, current_user)
+        execute_hooks(milestone, 'close')
       end
 
       milestone
diff --git a/app/services/milestones/create_service.rb b/app/services/milestones/create_service.rb
index e8a14adc10dd2dc4322a64ea8488f99d0c74f941..d8a9be6aafebd451d58d26b0e4dac489caba9118 100644
--- a/app/services/milestones/create_service.rb
+++ b/app/services/milestones/create_service.rb
@@ -9,6 +9,7 @@ def execute
 
       if milestone.save && milestone.project_milestone?
         event_service.open_milestone(milestone, current_user)
+        execute_hooks(milestone, 'create')
       end
 
       milestone
diff --git a/app/services/milestones/destroy_service.rb b/app/services/milestones/destroy_service.rb
index 6966764634f1ecaea43c2258fcd2c356ffe13d9a..dc605472e37bf0b10b820376c31c9baef1a6a0fe 100644
--- a/app/services/milestones/destroy_service.rb
+++ b/app/services/milestones/destroy_service.rb
@@ -14,6 +14,7 @@ def execute(milestone)
 
       return unless milestone.destroyed?
 
+      execute_hooks(milestone, 'delete') if milestone.project_milestone?
       milestone
     end
 
diff --git a/app/services/milestones/reopen_service.rb b/app/services/milestones/reopen_service.rb
index 125a3ec1367654c174f5f1c8d0b810653cb1f2c8..a4a9ccfd4011dc5371cea76c465bbabd59f6cebb 100644
--- a/app/services/milestones/reopen_service.rb
+++ b/app/services/milestones/reopen_service.rb
@@ -5,6 +5,7 @@ class ReopenService < Milestones::BaseService
     def execute(milestone)
       if milestone.activate && milestone.project_milestone?
         event_service.reopen_milestone(milestone, current_user)
+        execute_hooks(milestone, 'reopen')
       end
 
       milestone
diff --git a/app/services/test_hooks/project_service.rb b/app/services/test_hooks/project_service.rb
index b183210edb3b547c75716d32d0004769330eb4d5..25c81c3a49890775975ea787ce9cc5256dc2ac5e 100644
--- a/app/services/test_hooks/project_service.rb
+++ b/app/services/test_hooks/project_service.rb
@@ -35,6 +35,8 @@ def data
           wiki_page_events_data
         when 'releases_events'
           releases_events_data
+        when 'milestone_events'
+          milestone_events_data
         when 'emoji_events'
           emoji_events_data
         when 'resource_access_token_events'
diff --git a/app/views/shared/web_hooks/_form.html.haml b/app/views/shared/web_hooks/_form.html.haml
index a3c503c9efcdc4e5c27f1b57a3f5046c7a9c8dca..3be7461410d8bbb49eedb28f255d4290255f872b 100644
--- a/app/views/shared/web_hooks/_form.html.haml
+++ b/app/views/shared/web_hooks/_form.html.haml
@@ -58,6 +58,10 @@
       = form.gitlab_ui_checkbox_component :releases_events,
           integration_webhook_event_human_name(:releases_events),
           help_text: s_('Webhooks|A release is created, updated, or deleted.')
+    %li.gl-pb-3
+      = form.gitlab_ui_checkbox_component :milestone_events,
+          integration_webhook_event_human_name(:milestone_events),
+          help_text: s_('Webhooks|A milestone is created, closed, reopened, or deleted.')
     %li.gl-pb-3
       - emoji_help_link = link_to s_('Which emoji events trigger webhooks'), help_page_path('user/project/integrations/webhook_events.md', anchor: 'emoji-events')
       = form.gitlab_ui_checkbox_component :emoji_events,
diff --git a/db/migrate/20250613162310_add_milestone_events_to_web_hooks.rb b/db/migrate/20250613162310_add_milestone_events_to_web_hooks.rb
new file mode 100644
index 0000000000000000000000000000000000000000..0357de580ff5185decdac8faef3f76a53403bb57
--- /dev/null
+++ b/db/migrate/20250613162310_add_milestone_events_to_web_hooks.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class AddMilestoneEventsToWebHooks < Gitlab::Database::Migration[2.3]
+  milestone '18.2'
+
+  def change
+    add_column :web_hooks, :milestone_events, :boolean, null: false, default: false
+  end
+end
diff --git a/db/schema_migrations/20250613162310 b/db/schema_migrations/20250613162310
new file mode 100644
index 0000000000000000000000000000000000000000..16d68416d52b0133a6261f914aa49be9456c5847
--- /dev/null
+++ b/db/schema_migrations/20250613162310
@@ -0,0 +1 @@
+ecd7624a83f30d66eb361cdfa56d857162c5ff00a2f9ae3ce256013f003f4d1b
\ No newline at end of file
diff --git a/db/structure.sql b/db/structure.sql
index 02d98b13a2c0226ca680234760e5f266a0324fd5..86bb2cbe6f4c9c628522d3d550b4ed1f2b0ec56f 100644
--- a/db/structure.sql
+++ b/db/structure.sql
@@ -26011,6 +26011,7 @@ CREATE TABLE web_hooks (
     project_events boolean DEFAULT false NOT NULL,
     vulnerability_events boolean DEFAULT false NOT NULL,
     member_approval_events boolean DEFAULT false NOT NULL,
+    milestone_events boolean DEFAULT false NOT NULL,
     CONSTRAINT check_1e4d5cbdc5 CHECK ((char_length(name) <= 255)),
     CONSTRAINT check_23a96ad211 CHECK ((char_length(description) <= 2048)),
     CONSTRAINT check_69ef76ee0c CHECK ((char_length(custom_webhook_template) <= 4096))
diff --git a/doc/api/openapi/openapi_v2.yaml b/doc/api/openapi/openapi_v2.yaml
index d9675e4d8daaef41e4fbb89aa3011ef35370c40f..fbf92bfa947b448c12fcca8cc56fe378d778bb08 100644
--- a/doc/api/openapi/openapi_v2.yaml
+++ b/doc/api/openapi/openapi_v2.yaml
@@ -29125,6 +29125,7 @@ paths:
         - issues_events
         - job_events
         - merge_requests_events
+        - milestone_events
         - note_events
         - pipeline_events
         - push_events
@@ -59246,6 +59247,8 @@ definitions:
         type: boolean
       releases_events:
         type: boolean
+      milestone_events:
+        type: boolean
       emoji_events:
         type: boolean
       resource_access_token_events:
@@ -59304,6 +59307,9 @@ definitions:
       releases_events:
         type: boolean
         description: Trigger hook on release events
+      milestone_events:
+        type: boolean
+        description: Trigger hook on milestone events
       emoji_events:
         type: boolean
         description: Trigger hook on emoji events
@@ -59421,6 +59427,9 @@ definitions:
       releases_events:
         type: boolean
         description: Trigger hook on release events
+      milestone_events:
+        type: boolean
+        description: Trigger hook on milestone events
       emoji_events:
         type: boolean
         description: Trigger hook on emoji events
diff --git a/doc/api/project_webhooks.md b/doc/api/project_webhooks.md
index 0918a17b4620176fb20172764ce0d7de07d7c730..52279498140225aa923ead774beb6a59ba3e60df 100644
--- a/doc/api/project_webhooks.md
+++ b/doc/api/project_webhooks.md
@@ -70,6 +70,7 @@ Example response:
   "wiki_page_events": true,
   "deployment_events": true,
   "releases_events": true,
+  "milestone_events": true,
   "feature_flag_events": true,
   "enable_ssl_verification": true,
   "repository_update_events": false,
@@ -431,6 +432,7 @@ Supported attributes:
 | `branch_filter_strategy`       | string            | No       | Filter push events by branch. Possible values are `wildcard` (default), `regex`, and `all_branches`. |
 | `push_events`                  | boolean           | No       | Trigger project webhook on push events. |
 | `releases_events`              | boolean           | No       | Trigger project webhook on release events. |
+| `milestone_events`             | boolean           | No       | Trigger project webhook on milestone events. |
 | `tag_push_events`              | boolean           | No       | Trigger project webhook on tag push events. |
 | `token`                        | string            | No       | Secret token to validate received payloads; the token isn't returned in the response. |
 | `wiki_page_events`             | boolean           | No       | Trigger project webhook on wiki events. |
@@ -475,6 +477,7 @@ Supported attributes:
 | `branch_filter_strategy`       | string            | No       | Filter push events by branch. Possible values are `wildcard` (default), `regex`, and `all_branches`. |
 | `push_events`                  | boolean           | No       | Trigger project webhook on push events. |
 | `releases_events`              | boolean           | No       | Trigger project webhook on release events. |
+| `milestone_events`             | boolean           | No       | Trigger project webhook on milestone events. |
 | `tag_push_events`              | boolean           | No       | Trigger project webhook on tag push events. |
 | `token`                        | string            | No       | Secret token to validate received payloads. Not returned in the response. When you change the webhook URL, the secret token is reset and not retained. |
 | `wiki_page_events`             | boolean           | No       | Trigger project webhook on wiki page events. |
@@ -531,7 +534,7 @@ Supported attributes:
 |:----------|:------------------|:---------|:------------|
 | `hook_id` | integer           | Yes      | ID of the project webhook. |
 | `id`      | integer or string | Yes      | ID or [URL-encoded path of the project](rest/_index.md#namespaced-paths). |
-| `trigger` | string            | Yes      | One of `push_events`, `tag_push_events`, `issues_events`, `confidential_issues_events`, `note_events`, `merge_requests_events`, `job_events`, `pipeline_events`, `wiki_page_events`, `releases_events`, `emoji_events`, or `resource_access_token_events`. |
+| `trigger` | string            | Yes      | One of `push_events`, `tag_push_events`, `issues_events`, `confidential_issues_events`, `note_events`, `merge_requests_events`, `job_events`, `pipeline_events`, `wiki_page_events`, `releases_events`, `milestone_events`, `emoji_events`, or `resource_access_token_events`. |
 
 Example response:
 
diff --git a/doc/user/project/integrations/webhook_events.md b/doc/user/project/integrations/webhook_events.md
index f8b86b3ca97dd9afb0648af1935a29f7c62b3506..c5efc1b0621521e1a4e97095e14d644b16174281 100644
--- a/doc/user/project/integrations/webhook_events.md
+++ b/doc/user/project/integrations/webhook_events.md
@@ -37,6 +37,7 @@ Event type                                   | Trigger
 [Deployment event](#deployment-events)       | A deployment starts, succeeds, fails, or is canceled.
 [Feature flag event](#feature-flag-events)   | A feature flag is turned on or off.
 [Release event](#release-events)             | A release is created, edited, or deleted.
+[Milestone event](#milestone-events)         | A milestone is created, closed, reopened, or deleted.
 [Emoji event](#emoji-events)                 | An emoji reaction is added or removed.
 [Project or group access token event](#project-and-group-access-token-events) | A project or group access token will expire in seven days.
 [Vulnerability event](#vulnerability-events) | A vulnerability is created or updated.
@@ -2137,6 +2138,69 @@ Payload example:
 }
 ```
 
+## Milestone events
+
+{{< history >}}
+
+- [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/14213) in GitLab 18.2.
+
+{{< /history >}}
+
+Milestone events are triggered when a milestone is created, closed, reopened, or deleted.
+
+The available values for `object_attributes.action` in the payload are:
+
+- `create`
+- `close`
+- `reopen`
+
+Request header:
+
+```plaintext
+X-Gitlab-Event: Milestone Hook
+```
+
+Payload example:
+
+```json
+{
+  "object_kind": "milestone",
+  "event_type": "milestone",
+  "project": {
+    "id": 1,
+    "name": "Gitlab Test",
+    "description": "Aut reprehenderit ut est.",
+    "web_url": "http://example.com/gitlabhq/gitlab-test",
+    "avatar_url": null,
+    "git_ssh_url": "git@example.com:gitlabhq/gitlab-test.git",
+    "git_http_url": "http://example.com/gitlabhq/gitlab-test.git",
+    "namespace": "GitlabHQ",
+    "visibility_level": 20,
+    "path_with_namespace": "gitlabhq/gitlab-test",
+    "default_branch": "master",
+    "ci_config_path": null,
+    "homepage": "http://example.com/gitlabhq/gitlab-test",
+    "url": "http://example.com/gitlabhq/gitlab-test.git",
+    "ssh_url": "git@example.com:gitlabhq/gitlab-test.git",
+    "http_url": "http://example.com/gitlabhq/gitlab-test.git"
+  },
+  "object_attributes": {
+    "id": 61,
+    "iid": 10,
+    "title": "v1.0",
+    "description": "First stable release",
+    "state": "active",
+    "created_at": "2025-06-16 14:10:57 UTC",
+    "updated_at": "2025-06-16 14:10:57 UTC",
+    "due_date": "2025-06-30",
+    "start_date": "2025-06-16",
+    "group_id": null,
+    "project_id": 1
+  },
+  "action": "create"
+}
+```
+
 ## Emoji events
 
 {{< history >}}
diff --git a/lib/api/entities/project_hook.rb b/lib/api/entities/project_hook.rb
index a1ede6fe277176cc5cbc1d5dbf16029ec2212dee..b81720fa28f7fb38964332cda75c41ad423618b1 100644
--- a/lib/api/entities/project_hook.rb
+++ b/lib/api/entities/project_hook.rb
@@ -14,6 +14,7 @@ class ProjectHook < Hook
       expose :feature_flag_events, documentation: { type: 'boolean' }
       expose :job_events, documentation: { type: 'boolean' }
       expose :releases_events, documentation: { type: 'boolean' }
+      expose :milestone_events, documentation: { type: 'boolean' }
       expose :emoji_events, documentation: { type: 'boolean' }
       expose :resource_access_token_events, documentation: { type: 'boolean' }
       expose :vulnerability_events, documentation: { type: 'boolean' }
diff --git a/lib/api/project_hooks.rb b/lib/api/project_hooks.rb
index ecd32e9155f84925e4baf47e0345de1b75980969..ceffa33a63dcc32093d2104e81267bd04224ad44 100644
--- a/lib/api/project_hooks.rb
+++ b/lib/api/project_hooks.rb
@@ -37,6 +37,7 @@ def hook_scope
         optional :deployment_events, type: Boolean, desc: "Trigger hook on deployment events"
         optional :feature_flag_events, type: Boolean, desc: "Trigger hook on feature flag events"
         optional :releases_events, type: Boolean, desc: "Trigger hook on release events"
+        optional :milestone_events, type: Boolean, desc: "Trigger hook on milestone events"
         optional :emoji_events, type: Boolean, desc: "Trigger hook on emoji events"
         optional :resource_access_token_events, type: Boolean, desc: "Trigger hook on project access token expiry events"
         optional :enable_ssl_verification, type: Boolean, desc: "Do SSL verification when triggering the hook"
diff --git a/lib/gitlab/data_builder/milestone.rb b/lib/gitlab/data_builder/milestone.rb
new file mode 100644
index 0000000000000000000000000000000000000000..6f6e269c082513e6703b85d72261c9b2827ab0c2
--- /dev/null
+++ b/lib/gitlab/data_builder/milestone.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module DataBuilder
+    module Milestone
+      extend self
+
+      SAMPLE_DATA = {
+        id: 1,
+        iid: 1,
+        title: 'Sample milestone',
+        description: 'Sample milestone description',
+        state: 'active',
+        created_at: Time.current,
+        updated_at: Time.current,
+        due_date: 1.week.from_now,
+        start_date: Time.current
+      }.freeze
+
+      def build(milestone, action)
+        {
+          object_kind: 'milestone',
+          event_type: 'milestone',
+          project: milestone.project&.hook_attrs,
+          object_attributes: milestone.hook_attrs,
+          action: action
+        }
+      end
+
+      def build_sample(project)
+        milestone = project.milestones.first || ::Milestone.new(SAMPLE_DATA.merge(project: project))
+        build(milestone, 'create')
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/hook_data/milestone_builder.rb b/lib/gitlab/hook_data/milestone_builder.rb
new file mode 100644
index 0000000000000000000000000000000000000000..f2022a956eb25b685cf5a794ed87c340b7c5dda2
--- /dev/null
+++ b/lib/gitlab/hook_data/milestone_builder.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module HookData
+    class MilestoneBuilder < BaseBuilder
+      SAFE_HOOK_ATTRIBUTES = %i[
+        id
+        iid
+        title
+        description
+        state
+        created_at
+        updated_at
+        due_date
+        start_date
+        project_id
+      ].freeze
+
+      alias_method :milestone, :object
+
+      def build
+        milestone
+          .attributes
+          .with_indifferent_access
+          .slice(*SAFE_HOOK_ATTRIBUTES)
+      end
+    end
+  end
+end
diff --git a/locale/gitlab.pot b/locale/gitlab.pot
index d98c291e85dd788d6b31a5abae93f614c26fdd91..b1c1504ee5bea6274e2f3c41f2cb66b4082713b5 100644
--- a/locale/gitlab.pot
+++ b/locale/gitlab.pot
@@ -69501,6 +69501,9 @@ msgstr ""
 msgid "Webhooks|A merge request is created, updated, or merged."
 msgstr ""
 
+msgid "Webhooks|A milestone is created, closed, reopened, or deleted."
+msgstr ""
+
 msgid "Webhooks|A new tag is pushed to the repository."
 msgstr ""
 
diff --git a/spec/factories/project_hooks.rb b/spec/factories/project_hooks.rb
index 482cec1195d5cefc576907d38f773bfe66578adb..56959e051f828d81369062a07655e6ab621f77b5 100644
--- a/spec/factories/project_hooks.rb
+++ b/spec/factories/project_hooks.rb
@@ -30,6 +30,7 @@
       deployment_events { true }
       feature_flag_events { true }
       releases_events { true }
+      milestone_events { true }
       emoji_events { true }
       vulnerability_events { true }
     end
diff --git a/spec/features/projects/settings/webhooks_settings_spec.rb b/spec/features/projects/settings/webhooks_settings_spec.rb
index 062bcf907960c7e2cd4b4d917d769242d3b3678f..f1f824a8d657e4656f75bc154f944a38a8074623 100644
--- a/spec/features/projects/settings/webhooks_settings_spec.rb
+++ b/spec/features/projects/settings/webhooks_settings_spec.rb
@@ -43,6 +43,7 @@
         expect(page).to have_content('Comment')
         expect(page).to have_content('Merge request events')
         expect(page).to have_content('Pipeline events')
+        expect(page).to have_content('Milestone events')
         expect(page).to have_content('Wiki page events')
         expect(page).to have_content('Releases events')
         expect(page).to have_content('Emoji events')
diff --git a/spec/fixtures/api/schemas/public_api/v4/project_hook.json b/spec/fixtures/api/schemas/public_api/v4/project_hook.json
index 238f5839f77dfbbc46cec36b19723c4d4f27227d..fb9cc71728868576a5c514da83274b814159d04b 100644
--- a/spec/fixtures/api/schemas/public_api/v4/project_hook.json
+++ b/spec/fixtures/api/schemas/public_api/v4/project_hook.json
@@ -117,6 +117,9 @@
     "releases_events": {
       "type": "boolean"
     },
+    "milestone_events": {
+      "type": "boolean"
+    },
     "emoji_events": {
       "type": "boolean"
     },
diff --git a/spec/lib/gitlab/data_builder/milestone_spec.rb b/spec/lib/gitlab/data_builder/milestone_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..8eb13c8ac42811e692d15e5ebda2f4f673253037
--- /dev/null
+++ b/spec/lib/gitlab/data_builder/milestone_spec.rb
@@ -0,0 +1,130 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::DataBuilder::Milestone, feature_category: :team_planning do
+  let_it_be(:project) { create(:project) }
+
+  shared_examples 'builds milestone hook data' do
+    it { expect(data).to be_a(Hash) }
+
+    it 'includes the correct structure' do
+      expect(data[:object_kind]).to eq('milestone')
+      expect(data[:event_type]).to eq('milestone')
+      expect(data[:action]).to eq(action)
+    end
+  end
+
+  describe '.build' do
+    let(:milestone) { create(:milestone, project: project) }
+    let(:action) { 'create' }
+
+    subject(:data) { described_class.build(milestone, action) }
+
+    it_behaves_like 'builds milestone hook data'
+
+    it 'includes project data' do
+      expect(data[:project]).to eq(milestone.project.hook_attrs)
+    end
+
+    it 'includes milestone attributes' do
+      object_attributes = data[:object_attributes]
+
+      expect(object_attributes[:id]).to eq(milestone.id)
+      expect(object_attributes[:iid]).to eq(milestone.iid)
+      expect(object_attributes[:title]).to eq(milestone.title)
+      expect(object_attributes[:description]).to eq(milestone.description)
+      expect(object_attributes[:state]).to eq(milestone.state)
+      expect(object_attributes[:created_at]).to eq(milestone.created_at)
+      expect(object_attributes[:updated_at]).to eq(milestone.updated_at)
+      expect(object_attributes[:due_date]).to eq(milestone.due_date)
+      expect(object_attributes[:start_date]).to eq(milestone.start_date)
+      expect(object_attributes[:project_id]).to eq(milestone.project_id)
+    end
+
+    context 'with different actions' do
+      %w[create close reopen].each do |test_action|
+        context "when action is #{test_action}" do
+          let(:action) { test_action }
+
+          it "sets the action to #{test_action}" do
+            expect(data[:action]).to eq(test_action)
+          end
+        end
+      end
+    end
+
+    context 'with milestone having dates' do
+      let(:milestone) { create(:milestone, project: project, due_date: 1.week.from_now, start_date: 1.day.ago) }
+
+      it 'includes the date information' do
+        expect(data[:object_attributes][:due_date]).to eq(milestone.due_date)
+        expect(data[:object_attributes][:start_date]).to eq(milestone.start_date)
+      end
+    end
+
+    context 'with milestone having no dates' do
+      let(:milestone) { create(:milestone, project: project, due_date: nil, start_date: nil) }
+
+      it 'includes nil date information' do
+        expect(data[:object_attributes][:due_date]).to be_nil
+        expect(data[:object_attributes][:start_date]).to be_nil
+      end
+    end
+
+    context 'with closed milestone' do
+      let(:milestone) { create(:milestone, :closed, project: project) }
+
+      it 'includes the correct state' do
+        expect(data[:object_attributes][:state]).to eq('closed')
+      end
+    end
+
+    include_examples 'project hook data'
+  end
+
+  describe '.build_sample' do
+    let(:action) { 'create' }
+
+    context 'when project has existing milestones' do
+      subject(:data) { described_class.build_sample(project) }
+
+      let_it_be(:existing_milestone) { create(:milestone, project: project) }
+
+      it_behaves_like 'builds milestone hook data'
+
+      it 'includes project data' do
+        expect(data[:project]).to eq(project.hook_attrs)
+      end
+
+      it 'uses the first existing milestone' do
+        expect(data[:object_attributes][:id]).to eq(existing_milestone.id)
+        expect(data[:object_attributes][:title]).to eq(existing_milestone.title)
+      end
+
+      include_examples 'project hook data'
+    end
+
+    context 'when project has no milestones' do
+      subject(:data) { described_class.build_sample(clean_project) }
+
+      let_it_be(:clean_project) { create(:project) }
+
+      it_behaves_like 'builds milestone hook data'
+
+      it 'includes project data' do
+        expect(data[:project]).to eq(clean_project.hook_attrs)
+      end
+
+      it 'creates a sample milestone with predefined data' do
+        expect(data[:object_attributes][:title]).to eq('Sample milestone')
+        expect(data[:object_attributes][:description]).to eq('Sample milestone description')
+        expect(data[:object_attributes][:state]).to eq('active')
+      end
+
+      include_examples 'project hook data' do
+        let(:project) { clean_project }
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/hook_data/milestone_builder_spec.rb b/spec/lib/gitlab/hook_data/milestone_builder_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..ef26dc9dd5d339bd58a03c59b67bab931298da52
--- /dev/null
+++ b/spec/lib/gitlab/hook_data/milestone_builder_spec.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::HookData::MilestoneBuilder, feature_category: :webhooks do
+  let_it_be(:milestone) { create(:milestone) }
+
+  let(:builder) { described_class.new(milestone) }
+
+  describe '#build' do
+    subject(:data) { builder.build }
+
+    it 'includes safe attributes' do
+      expect(data.keys).to match_array(described_class::SAFE_HOOK_ATTRIBUTES.map(&:to_s))
+    end
+
+    it 'returns indifferent access hash' do
+      expect(data).to be_a(ActiveSupport::HashWithIndifferentAccess)
+    end
+
+    it 'includes correct milestone data' do
+      expect(data['id']).to eq(milestone.id)
+      expect(data['iid']).to eq(milestone.iid)
+      expect(data['title']).to eq(milestone.title)
+      expect(data['state']).to eq(milestone.state)
+      expect(data['project_id']).to eq(milestone.project_id)
+    end
+  end
+end
diff --git a/spec/lib/gitlab/import_export/project/relation_factory_spec.rb b/spec/lib/gitlab/import_export/project/relation_factory_spec.rb
index f898b18e20035dbb6d3aeb87294fe92e55ec8f59..439fedd3243226ffe683122cd393370f9bab2532 100644
--- a/spec/lib/gitlab/import_export/project/relation_factory_spec.rb
+++ b/spec/lib/gitlab/import_export/project/relation_factory_spec.rb
@@ -67,6 +67,7 @@ def values
         'job_events' => false,
         'wiki_page_events' => true,
         'releases_events' => false,
+        'milestone_events' => false,
         'emoji_events' => false,
         'resource_access_token_events' => false,
         'token' => token
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index 516c94336f4ed4563cc8f80fa32888d6f6f29a05..a85a4805f0c27c72b271b9b56768cb9505fe4736 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -619,6 +619,7 @@ ProjectHook:
 - confidential_note_events
 - repository_update_events
 - releases_events
+- milestone_events
 - emoji_events
 - resource_access_token_events
 ProtectedBranch:
diff --git a/spec/services/integrations/test/project_service_spec.rb b/spec/services/integrations/test/project_service_spec.rb
index 4f8f932fb45c76e8f395e651d301342f023a4bbb..cc88a815f65b45349d8fa2d7d297977c9bc66f25 100644
--- a/spec/services/integrations/test/project_service_spec.rb
+++ b/spec/services/integrations/test/project_service_spec.rb
@@ -41,6 +41,24 @@
         end
       end
 
+      context 'milestone' do
+        let(:event) { 'milestone' }
+
+        before do
+          # Mock the integration to support milestone events for testing
+          allow(integration).to receive(:supported_events).and_return(integration.supported_events + ['milestone'])
+        end
+
+        it 'executes integration' do
+          milestone = create(:milestone, project: project)
+          allow(Gitlab::DataBuilder::Milestone).to receive(:build).and_return(sample_data)
+          allow_next(MilestonesFinder).to receive(:execute).and_return([milestone])
+
+          expect(integration).to receive(:test).with(sample_data).and_return(success_result)
+          expect(subject).to eq(success_result)
+        end
+      end
+
       context 'push' do
         let(:event) { 'push' }
 
diff --git a/spec/services/milestones/close_service_spec.rb b/spec/services/milestones/close_service_spec.rb
index f362c8da6423d8a8dd28bba579809fcf4713e2c2..2c0caa38bb6ec3436612cffb0af59fd218f654da 100644
--- a/spec/services/milestones/close_service_spec.rb
+++ b/spec/services/milestones/close_service_spec.rb
@@ -12,19 +12,71 @@
   end
 
   describe '#execute' do
-    before do
-      described_class.new(project, user, {}).execute(milestone)
+    let(:service) { described_class.new(project, user, {}) }
+
+    context 'when service is called before test suite' do
+      before do
+        service.execute(milestone)
+      end
+
+      it { expect(milestone).to be_valid }
+      it { expect(milestone).to be_closed }
+
+      describe 'event' do
+        let(:event) { Event.recent.first }
+
+        it { expect(event.milestone).to be_truthy }
+        it { expect(event.target).to eq(milestone) }
+        it { expect(event.action_name).to eq('closed') }
+      end
     end
 
-    it { expect(milestone).to be_valid }
-    it { expect(milestone).to be_closed }
+    shared_examples 'closes the milestone' do |with_project_hooks:|
+      it 'executes hooks with close action and creates new event' do
+        expect(service).to receive(:execute_hooks).with(milestone, 'close').and_call_original
+        expect(project).to receive(:execute_hooks).with(kind_of(Hash), :milestone_hooks) if with_project_hooks
+
+        expect { service.execute(milestone) }.to change { Event.count }.by(1)
+      end
+    end
+
+    shared_examples 'does not close the milestone' do
+      it 'does not execute hooks and does not create new event' do
+        expect(service).not_to receive(:execute_hooks)
+
+        expect { service.execute(milestone) }.not_to change { Event.count }
+      end
+    end
+
+    context 'when milestone is successfully closed' do
+      context 'when project has active milestone hooks' do
+        let(:project) do
+          create(:project).tap do |project|
+            create(:project_hook, project: project, milestone_events: true)
+          end
+        end
+
+        it_behaves_like 'closes the milestone', with_project_hooks: true
+      end
+
+      context 'when project has no active milestone hooks' do
+        it_behaves_like 'closes the milestone', with_project_hooks: false
+      end
+    end
+
+    context 'when milestone fails to close' do
+      context 'when milestone is already closed' do
+        let(:milestone) { create(:milestone, :closed, project: project) }
+
+        it_behaves_like 'does not close the milestone'
+      end
 
-    describe 'event' do
-      let(:event) { Event.recent.first }
+      context 'when milestone is a group milestone' do
+        let(:group) { create(:group) }
+        let(:milestone) { create(:milestone, group: group) }
 
-      it { expect(event.milestone).to be_truthy }
-      it { expect(event.target).to eq(milestone) }
-      it { expect(event.action_name).to eq('closed') }
+        it_behaves_like 'does not close the milestone'
+      end
     end
   end
 end
diff --git a/spec/services/milestones/create_service_spec.rb b/spec/services/milestones/create_service_spec.rb
index 70010d88fbd3b319bb9b91b7d5853f9daefd9bc3..1999b1d40392bb7befdea7614f188a0194150a80 100644
--- a/spec/services/milestones/create_service_spec.rb
+++ b/spec/services/milestones/create_service_spec.rb
@@ -29,6 +29,29 @@
         expect(milestone.title).to eq('New Milestone')
         expect(milestone.description).to eq('Description')
       end
+
+      shared_examples 'creates the milestone' do |with_project_hooks:|
+        it 'executes hooks with create action and creates new event' do
+          expect(create_milestone).to receive(:execute_hooks).with(kind_of(Milestone), 'create').and_call_original
+          expect(project).to receive(:execute_hooks).with(kind_of(Hash), :milestone_hooks) if with_project_hooks
+
+          expect { create_milestone.execute }.to change { Event.count }.by(1)
+        end
+      end
+
+      context 'when project has active milestone hooks' do
+        let(:project) do
+          create(:project).tap do |project|
+            create(:project_hook, project: project, milestone_events: true)
+          end
+        end
+
+        it_behaves_like 'creates the milestone', with_project_hooks: true
+      end
+
+      context 'when project has no active milestone hooks' do
+        it_behaves_like 'creates the milestone', with_project_hooks: false
+      end
     end
 
     context 'when milestone fails to save' do
@@ -48,6 +71,12 @@
         create_milestone.execute
       end
 
+      it 'does not execute hooks and does not create new event' do
+        expect(create_milestone).not_to receive(:execute_hooks)
+
+        expect { create_milestone.execute }.not_to change { Event.count }
+      end
+
       it 'returns the unsaved milestone' do
         milestone = create_milestone.execute
         expect(milestone).to be_a(Milestone)
@@ -56,6 +85,21 @@
       end
     end
 
+    context 'when milestone is a group milestone' do
+      let(:group) { create(:group) }
+      let(:group_service) { described_class.new(group, user, params) }
+
+      it 'does not execute hooks for group milestones' do
+        milestone = build(:milestone, group: group)
+        allow(milestone).to receive(:save).and_return(true)
+        allow(group_service).to receive(:build_milestone).and_return(milestone)
+
+        expect(group).not_to receive(:execute_hooks)
+
+        group_service.execute
+      end
+    end
+
     it 'calls before_create method' do
       expect(create_milestone).to receive(:before_create)
       create_milestone.execute
diff --git a/spec/services/milestones/destroy_service_spec.rb b/spec/services/milestones/destroy_service_spec.rb
index fd276a54e10111dbea5b79ae41f566a723419317..c76edd2648c361276265637c8326268b6233e839 100644
--- a/spec/services/milestones/destroy_service_spec.rb
+++ b/spec/services/milestones/destroy_service_spec.rb
@@ -48,7 +48,9 @@
 
       it_behaves_like 'deletes milestone id from issuables'
 
-      it 'logs destroy event' do
+      it 'logs destroy event and runs on-delete webhook' do
+        expect(service).to receive(:execute_hooks).with(milestone, 'delete')
+
         service.execute(milestone)
 
         event = Event.where(project_id: milestone.project_id, target_type: 'Milestone')
@@ -84,7 +86,9 @@
 
       it_behaves_like 'deletes milestone id from issuables'
 
-      it 'does not log destroy event' do
+      it 'does not log destroy event and does not run on-delete webhook' do
+        expect(service).not_to receive(:execute_hooks).with(milestone, 'delete')
+
         expect { service.execute(milestone) }.not_to change { Event.count }
       end
     end
diff --git a/spec/services/milestones/reopen_service_spec.rb b/spec/services/milestones/reopen_service_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..fc7874cbc84fb5a74965806bc0821a6dd65faec4
--- /dev/null
+++ b/spec/services/milestones/reopen_service_spec.rb
@@ -0,0 +1,85 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Milestones::ReopenService, feature_category: :team_planning do
+  let_it_be(:project) { create(:project) }
+  let_it_be(:user) { create(:user) }
+
+  let(:milestone) { create(:milestone, :closed, title: "Milestone v1.2", project: project) }
+
+  before_all do
+    project.add_maintainer(user)
+  end
+
+  describe '#execute' do
+    let(:service) { described_class.new(project, user, {}) }
+
+    context 'when service is called before test suite' do
+      before do
+        service.execute(milestone)
+      end
+
+      it { expect(milestone).to be_valid }
+      it { expect(milestone).to be_active }
+
+      describe 'event' do
+        let(:event) { Event.recent.first }
+
+        it { expect(event.milestone).to be_truthy }
+        it { expect(event.target).to eq(milestone) }
+        it { expect(event.action_name).to eq('opened') }
+      end
+    end
+
+    shared_examples 'reopens the milestone' do |with_project_hooks:|
+      it 'executes hooks with reopen action and creates new event' do
+        expect(service).to receive(:execute_hooks).with(milestone, 'reopen').and_call_original
+        expect(project).to receive(:execute_hooks).with(kind_of(Hash), :milestone_hooks) if with_project_hooks
+
+        expect { service.execute(milestone) }.to change { Event.count }.by(1)
+      end
+    end
+
+    shared_examples 'does not reopen the milestone' do
+      it 'does not execute hooks and does not create new event' do
+        expect(service).not_to receive(:execute_hooks)
+
+        expect { service.execute(milestone) }.not_to change { Event.count }
+      end
+    end
+
+    context 'when milestone is successfully reopened' do
+      let(:milestone) { create(:milestone, :closed, project: project) }
+
+      context 'when project has active milestone hooks' do
+        let(:project) do
+          create(:project).tap do |project|
+            create(:project_hook, project: project, milestone_events: true)
+          end
+        end
+
+        it_behaves_like 'reopens the milestone', with_project_hooks: true
+      end
+
+      context 'when project has no active milestone hooks' do
+        it_behaves_like 'reopens the milestone', with_project_hooks: false
+      end
+    end
+
+    context 'when milestone fails to reopen' do
+      context 'when milestone is already active' do
+        let(:milestone) { create(:milestone, project: project) }
+
+        it_behaves_like 'does not reopen the milestone'
+      end
+
+      context 'when milestone is a group milestone' do
+        let(:group) { create(:group) }
+        let(:milestone) { create(:milestone, :closed, group: group) }
+
+        it_behaves_like 'does not reopen the milestone'
+      end
+    end
+  end
+end
diff --git a/spec/services/test_hooks/project_service_spec.rb b/spec/services/test_hooks/project_service_spec.rb
index 1e06911f8f799cb2dd1613fc5519420e9ccd2b66..cd2bc14c0edeca1be9ef37e29d92e5fd48433161 100644
--- a/spec/services/test_hooks/project_service_spec.rb
+++ b/spec/services/test_hooks/project_service_spec.rb
@@ -211,6 +211,18 @@
       end
     end
 
+    context 'milestone_events' do
+      let(:trigger) { 'milestone_events' }
+      let(:trigger_key) { :milestone_hooks }
+
+      it 'executes hook' do
+        allow(Gitlab::DataBuilder::Milestone).to receive(:build_sample).and_return(sample_data)
+
+        expect(hook).to receive(:execute).with(sample_data, trigger_key, force: true).and_return(success_result)
+        expect(service.execute).to include(success_result)
+      end
+    end
+
     context 'emoji' do
       let(:trigger) { 'emoji_events' }
       let(:trigger_key) { :emoji_hooks }