diff --git a/db/docs/batched_background_migrations/backfill_missing_namespace_id_on_notes.yml b/db/docs/batched_background_migrations/backfill_missing_namespace_id_on_notes.yml
new file mode 100644
index 0000000000000000000000000000000000000000..c15162fbf4a839b4587a03161f0c9b5862a28321
--- /dev/null
+++ b/db/docs/batched_background_migrations/backfill_missing_namespace_id_on_notes.yml
@@ -0,0 +1,9 @@
+---
+migration_job_name: BackfillMissingNamespaceIdOnNotes
+description: Backfills missing namespace_id values to support sharding
+feature_category: code_review_workflow
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163687
+milestone: '17.9'
+queued_migration_version: 20240822220027
+finalize_after: "2025-03-31"
+finalized_by: # version of the migration that finalized this BBM
diff --git a/db/post_migrate/20240822220027_queue_backfill_missing_namespace_id_on_notes.rb b/db/post_migrate/20240822220027_queue_backfill_missing_namespace_id_on_notes.rb
new file mode 100644
index 0000000000000000000000000000000000000000..c83f1a97a46b16fa3c0d9917de46587cc6995a6d
--- /dev/null
+++ b/db/post_migrate/20240822220027_queue_backfill_missing_namespace_id_on_notes.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+# See https://docs.gitlab.com/ee/development/database/batched_background_migrations.html
+# for more information on when/how to queue batched background migrations
+
+# Update below commented lines with appropriate values.
+
+class QueueBackfillMissingNamespaceIdOnNotes < Gitlab::Database::Migration[2.2]
+  milestone '17.5'
+
+  restrict_gitlab_migration gitlab_schema: :gitlab_main
+
+  MIGRATION = "BackfillMissingNamespaceIdOnNotes"
+  DELAY_INTERVAL = 2.minutes
+  BATCH_SIZE = 1000
+  SUB_BATCH_SIZE = 100
+  GITLAB_OPTIMIZED_BATCH_SIZE = 75_000
+  GITLAB_OPTIMIZED_SUB_BATCH_SIZE = 250
+
+  def up
+    queue_batched_background_migration(
+      MIGRATION,
+      :notes,
+      :id,
+      job_interval: DELAY_INTERVAL,
+      **batch_sizes
+    )
+  end
+
+  def down
+    delete_batched_background_migration(MIGRATION, :notes, :id, [])
+  end
+
+  private
+
+  def batch_sizes
+    if Gitlab.com_except_jh?
+      {
+        batch_size: GITLAB_OPTIMIZED_BATCH_SIZE,
+        sub_batch_size: GITLAB_OPTIMIZED_SUB_BATCH_SIZE
+      }
+    else
+      {
+        batch_size: BATCH_SIZE,
+        sub_batch_size: SUB_BATCH_SIZE
+      }
+    end
+  end
+end
diff --git a/db/schema_migrations/20240822220027 b/db/schema_migrations/20240822220027
new file mode 100644
index 0000000000000000000000000000000000000000..d078b0a58951681848ceda6b6e1e517637cd6a0d
--- /dev/null
+++ b/db/schema_migrations/20240822220027
@@ -0,0 +1 @@
+dff289034006ffbf12d45275cf91d3dc3843c80894cea79738ffec3149edbf76
\ No newline at end of file
diff --git a/lib/gitlab/background_migration/backfill_missing_namespace_id_on_notes.rb b/lib/gitlab/background_migration/backfill_missing_namespace_id_on_notes.rb
new file mode 100644
index 0000000000000000000000000000000000000000..58e29346a3a8d7ec8fefb0a0ad4d2c401ec11a7c
--- /dev/null
+++ b/lib/gitlab/background_migration/backfill_missing_namespace_id_on_notes.rb
@@ -0,0 +1,93 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    class BackfillMissingNamespaceIdOnNotes < BatchedMigrationJob
+      operation_name :backfill_missing_namespace_id_on_notes
+      feature_category :code_review_workflow
+
+      def perform
+        each_sub_batch do |sub_batch|
+          Gitlab::Database.allow_cross_joins_across_databases(
+            url: 'https://gitlab.com/gitlab-org/gitlab/-/merge_requests/163687'
+          ) do
+            connection.execute(build_query(sub_batch))
+          end
+        end
+      end
+
+      private
+
+      # rubocop:disable Layout/LineLength -- SQL!
+      # rubocop:disable Metrics/MethodLength -- I do what I want
+      def build_query(scope)
+        records_query = scope.where(namespace_id: nil).select("
+          id,
+          (
+            coalesce(
+            (case
+              when exists (select 1 from projects where id = notes.project_id) then (select namespace_id from projects where id = notes.project_id)
+              when noteable_type  = 'AlertManagement::Alert' then (select namespace_id from projects where id = (select project_id from alert_management_alerts where noteable_id = notes.id limit 1) limit 1)
+              when noteable_type  = 'MergeRequest' then (select namespace_id from projects where id = (select project_id from merge_requests where noteable_id = notes.id limit 1) limit 1)
+              when noteable_type  = 'Vulnerability' then (select namespace_id from projects where id = (select project_id from vulnerabilities where noteable_id = notes.id limit 1) limit 1)
+              -- These 2 need to pull namespace_id from the noteable
+              when noteable_type  = 'DesignManagement::Design' then (select namespace_id from design_management_designs where id = notes.noteable_id limit 1)
+              when noteable_type  = 'Issue' then (select namespace_id from issues where id = notes.noteable_id limit 1)
+              -- Epics pull in group_id
+              when noteable_type  = 'Epic' then (select group_id from epics where id = notes.noteable_id limit 1)
+              -- Snippets pull from author
+              when noteable_type  = 'Snippet' then (select id from namespaces where owner_id = (select author_id from notes where id = notes.id limit 1) limit 1)
+              -- Commits pull namespace_id from the project of the note
+              when noteable_type  = 'Commit' then (select namespace_id from projects where id = notes.project_id limit 1)
+            else
+              -1
+            end
+          ), -1)) as namespace_id_to_set
+        ")
+
+        <<~SQL
+        with records AS (
+          #{records_query.to_sql}
+        ), updated_rows as (
+          -- updating records with the located namespace_id_to_set value
+          update notes set namespace_id = namespace_id_to_set from records where records.id=notes.id and namespace_id_to_set <> -1
+        ), deleted_rows as (
+          -- deleting the records where we couldn't find the namespace id
+          delete from notes where id IN (select id from records where namespace_id_to_set = -1)
+        )
+        select 1
+        SQL
+      end
+      # rubocop:enable Layout/LineLength
+      # rubocop:enable Metrics/MethodLength
+
+      def backfillable?(note)
+        note.noteable_type.present?
+      end
+
+      def extract_namespace_id(note)
+        # Attempt to find namespace_id from the project first.
+        #
+        if note.project_id
+          project = Project.find_by_id(note.project_id)
+
+          return project.namespace_id if project
+        end
+
+        # We have to load the noteable here because we don't have access to the
+        #   usual ActiveRecord relationships to do it for us.
+        #
+        noteable = note.noteable_type.constantize.find(note.noteable_id)
+
+        case note.noteable_type
+        when "AlertManagement::Alert", "Commit", "MergeRequest", "Vulnerability"
+          noteable.project.namespace_id
+        when "DesignManagement::Design", "Epic", "Issue"
+          noteable.namespace_id
+        when "Snippet"
+          noteable.author.namespace_id
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/background_migration/backfill_missing_namespace_id_on_notes_spec.rb b/spec/lib/gitlab/background_migration/backfill_missing_namespace_id_on_notes_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..11f0aa31197fa77dc07b1d8caee3fbdf5f730e37
--- /dev/null
+++ b/spec/lib/gitlab/background_migration/backfill_missing_namespace_id_on_notes_spec.rb
@@ -0,0 +1,219 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::BackgroundMigration::BackfillMissingNamespaceIdOnNotes, feature_category: :code_review_workflow do
+  let(:namespaces_table) { table(:namespaces) }
+  let(:notes_table) { table(:notes) }
+  let(:projects_table) { table(:projects) }
+  let(:snippets_table) { table(:snippets) }
+  let(:users_table) { table(:users) }
+  let(:epics_table) { table(:epics) }
+  let(:issues_table) { table(:issues) }
+  let(:work_item_types_table) { table(:work_item_types) }
+  let(:organizations_table) { table(:organizations) }
+
+  let!(:organization) { organizations_table.create!(name: 'organization', path: 'organization') }
+
+  let(:namespace_1) do
+    namespaces_table.create!(
+      name: 'namespace',
+      path: 'namespace-path-1',
+      organization_id: organization.id
+    )
+  end
+
+  let(:project_namespace_2) do
+    namespaces_table.create!(
+      name: 'namespace',
+      path: 'namespace-path-2',
+      type: 'Project',
+      organization_id: organization.id
+    )
+  end
+
+  let!(:project_1) do
+    projects_table
+    .create!(
+      name: 'project1',
+      path: 'path1',
+      namespace_id: namespace_1.id,
+      project_namespace_id: project_namespace_2.id,
+      visibility_level: 0,
+      organization_id: organization.id
+    )
+  end
+
+  let!(:user_1) { users_table.create!(name: 'bob', email: 'bob@example.com', projects_limit: 1) }
+
+  context "when namespace_id is derived from note.project_id" do
+    let(:alert_management_alert_note) do
+      notes_table.create!(project_id: project_1.id, noteable_type: "AlertManagement::Alert")
+    end
+
+    let(:commit_note) { notes_table.create!(project_id: project_1.id, noteable_type: "Commit") }
+    let(:merge_request_note) { notes_table.create!(project_id: project_1.id, noteable_type: "MergeRequest") }
+    let(:vulnerability_note) { notes_table.create!(project_id: project_1.id, noteable_type: "Vulnerability") }
+    let(:design_note) { notes_table.create!(project_id: project_1.id, noteable_type: "Design") }
+    let(:work_item_note) { notes_table.create!(project_id: project_1.id, noteable_type: "WorkItem") }
+    let(:issue_note) { notes_table.create!(project_id: project_1.id, noteable_type: "Issue") }
+
+    it "updates the namespace_id" do
+      [
+        alert_management_alert_note,
+        commit_note,
+        merge_request_note,
+        vulnerability_note,
+        design_note,
+        work_item_note,
+        issue_note
+      ].each do |test_note|
+        expect(test_note.project_id).not_to be_nil
+
+        test_note.update_columns(namespace_id: nil)
+        test_note.reload
+
+        expect(test_note.namespace_id).to be_nil
+
+        described_class.new(
+          start_id: test_note.id,
+          end_id: test_note.id,
+          batch_table: :notes,
+          batch_column: :id,
+          sub_batch_size: 1,
+          pause_ms: 0,
+          connection: ActiveRecord::Base.connection
+        ).perform
+
+        test_note.reload
+
+        expect(test_note.namespace_id).not_to be_nil
+        expect(test_note.namespace_id).to eq(Project.find(test_note.project_id).namespace_id)
+      end
+    end
+  end
+
+  context "when namespace_id is derived from noteable.author.namespace_id" do
+    let!(:snippet) do
+      snippets_table.create!(
+        author_id: user_1.id,
+        project_id: project_1.id
+      )
+    end
+
+    let(:personal_snippet_note) do
+      notes_table.create!(author_id: user_1.id, noteable_type: "Snippet", noteable_id: snippet.id)
+    end
+
+    let(:project_snippet_note) do
+      notes_table.create!(author_id: user_1.id, noteable_type: "Snippet", noteable_id: snippet.id)
+    end
+
+    let!(:user_namespace) do
+      namespaces_table.create!(
+        name: 'namespace',
+        path: 'user-namespace-path',
+        type: 'User',
+        owner_id: user_1.id,
+        organization_id: organization.id
+      )
+    end
+
+    it "updates the namespace_id" do
+      [project_snippet_note, personal_snippet_note].each do |test_note|
+        test_note.update_columns(namespace_id: nil)
+        test_note.reload
+
+        expect(test_note.namespace_id).to be_nil
+
+        described_class.new(
+          start_id: test_note.id,
+          end_id: test_note.id,
+          batch_table: :notes,
+          batch_column: :id,
+          sub_batch_size: 1,
+          pause_ms: 0,
+          connection: ActiveRecord::Base.connection
+        ).perform
+
+        test_note.reload
+
+        expect(test_note.namespace_id).not_to be_nil
+        expect(test_note.namespace_id).to eq(user_namespace.id)
+      end
+    end
+  end
+
+  context "when namespace_id is derived from noteable.id" do
+    let!(:group_namespace) do
+      namespaces_table.create!(
+        name: 'namespace',
+        path: 'group-namespace-path',
+        type: 'Group',
+        owner_id: user_1.id,
+        organization_id: organization.id
+      )
+    end
+
+    let!(:work_items_type) do
+      work_item_types_table.create!(
+        id: Random.random_number(4000),
+        name: 'User Story',
+        correct_id: Random.random_number(4000)
+      )
+    end
+
+    let!(:issue) do
+      issues_table.create!(
+        title: "Example Epic",
+        author_id: user_1.id,
+        namespace_id: group_namespace.id,
+        correct_work_item_type_id: work_items_type.correct_id,
+        work_item_type_id: work_items_type.id
+      )
+    end
+
+    let!(:epic) do
+      epics_table.create!(
+        title: "Example Epic",
+        group_id: group_namespace.id,
+        author_id: user_1.id,
+        iid: Random.random_number(4000),
+        title_html: "<blink>Example</blink>",
+        issue_id: issue.id
+      )
+    end
+
+    let(:epic_note) do
+      notes_table.create!(
+        namespace_id: group_namespace.id,
+        noteable_type: "Epic",
+        noteable_id: epic.id
+      )
+    end
+
+    it "updates the namespace_id" do
+      [epic_note].each do |test_note|
+        test_note.update_columns(namespace_id: nil)
+        test_note.reload
+
+        expect(test_note.namespace_id).to be_nil
+
+        described_class.new(
+          start_id: test_note.id,
+          end_id: test_note.id,
+          batch_table: :notes,
+          batch_column: :id,
+          sub_batch_size: 1,
+          pause_ms: 0,
+          connection: ActiveRecord::Base.connection
+        ).perform
+
+        test_note.reload
+
+        expect(test_note.namespace_id).not_to be_nil
+        expect(test_note.namespace_id).to eq(group_namespace.id)
+      end
+    end
+  end
+end
diff --git a/spec/migrations/20240822220027_queue_backfill_missing_namespace_id_on_notes_spec.rb b/spec/migrations/20240822220027_queue_backfill_missing_namespace_id_on_notes_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..01c61445fe0ca4da704a73fd56aa2b15804ef079
--- /dev/null
+++ b/spec/migrations/20240822220027_queue_backfill_missing_namespace_id_on_notes_spec.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!
+
+RSpec.describe QueueBackfillMissingNamespaceIdOnNotes, feature_category: :code_review_workflow do
+  let!(:batched_migration) { described_class::MIGRATION }
+
+  it 'schedules a new batched migration' do
+    reversible_migration do |migration|
+      migration.before -> {
+        expect(batched_migration).not_to have_scheduled_batched_migration
+      }
+
+      migration.after -> {
+        expect(batched_migration).to have_scheduled_batched_migration(
+          table_name: :notes,
+          column_name: :id,
+          interval: described_class::DELAY_INTERVAL,
+          batch_size: described_class::BATCH_SIZE,
+          sub_batch_size: described_class::SUB_BATCH_SIZE
+        )
+      }
+    end
+  end
+end