diff --git a/db/docs/batched_background_migrations/backfill_user_agent_details_organization_id.yml b/db/docs/batched_background_migrations/backfill_user_agent_details_organization_id.yml
new file mode 100644
index 0000000000000000000000000000000000000000..fa05a4fe07c70420ae27ca75c9f4e2f8037962a4
--- /dev/null
+++ b/db/docs/batched_background_migrations/backfill_user_agent_details_organization_id.yml
@@ -0,0 +1,8 @@
+---
+migration_job_name: BackfillUserAgentDetailsOrganizationId
+description: Backfill the sharding key on the user_agent_details table
+feature_category: instance_resiliency
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/215327
+milestone: '18.8'
+queued_migration_version: 20251215170942
+finalized_by: # version of the migration that finalized this BBM
diff --git a/db/post_migrate/20251215170942_queue_backfill_user_agent_details_organization_id.rb b/db/post_migrate/20251215170942_queue_backfill_user_agent_details_organization_id.rb
new file mode 100644
index 0000000000000000000000000000000000000000..2dbae2a3ada5107fd0dcca3c92b31544b491d746
--- /dev/null
+++ b/db/post_migrate/20251215170942_queue_backfill_user_agent_details_organization_id.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class QueueBackfillUserAgentDetailsOrganizationId < Gitlab::Database::Migration[2.3]
+  milestone '18.8'
+
+  restrict_gitlab_migration gitlab_schema: :gitlab_main_org
+
+  MIGRATION = "BackfillUserAgentDetailsOrganizationId"
+  DELAY_INTERVAL = 2.minutes
+  BATCH_SIZE = 1_000
+  SUB_BATCH_SIZE = 100
+
+  def up
+    queue_batched_background_migration(
+      MIGRATION,
+      :user_agent_details,
+      :id,
+      job_interval: DELAY_INTERVAL,
+      batch_size: BATCH_SIZE,
+      sub_batch_size: SUB_BATCH_SIZE
+    )
+  end
+
+  def down
+    delete_batched_background_migration(MIGRATION, :user_agent_details, :id, [])
+  end
+end
diff --git a/db/schema_migrations/20251215170942 b/db/schema_migrations/20251215170942
new file mode 100644
index 0000000000000000000000000000000000000000..d5040f74eee713037413eb8bd1485fe9ac754492
--- /dev/null
+++ b/db/schema_migrations/20251215170942
@@ -0,0 +1 @@
+d46b4e76569d49fc319328e2a6fa6e4518e2cd816fecef517813701408825ffd
\ No newline at end of file
diff --git a/lib/gitlab/background_migration/backfill_user_agent_details_organization_id.rb b/lib/gitlab/background_migration/backfill_user_agent_details_organization_id.rb
new file mode 100644
index 0000000000000000000000000000000000000000..c4a5719dede5d3d3a7a664db6a8111f5b5c1cfcc
--- /dev/null
+++ b/lib/gitlab/background_migration/backfill_user_agent_details_organization_id.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    class BackfillUserAgentDetailsOrganizationId < BatchedMigrationJob
+      operation_name :backfill_user_agent_details_organization_id
+      feature_category :instance_resiliency
+
+      ORGANIZATION_ID = 1
+
+      def perform
+        each_sub_batch do |sub_batch|
+          sub_batch
+            .where(organization_id: nil)
+            .update_all(organization_id: ORGANIZATION_ID)
+        end
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/background_migration/backfill_user_agent_details_organization_id_spec.rb b/spec/lib/gitlab/background_migration/backfill_user_agent_details_organization_id_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..1589c4128a71ebc28d3915887fb46db78ca1a595
--- /dev/null
+++ b/spec/lib/gitlab/background_migration/backfill_user_agent_details_organization_id_spec.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::BackgroundMigration::BackfillUserAgentDetailsOrganizationId, feature_category: :instance_resiliency do
+  let(:connection) { ApplicationRecord.connection }
+
+  let(:issues) { table(:issues) }
+  let(:namespaces) { table(:namespaces) }
+  let(:organizations) { table(:organizations) }
+  let(:projects) { table(:projects) }
+  let(:user_agent_details) { table(:user_agent_details) { |t| t.belongs_to :subject, polymorphic: true } }
+
+  let!(:default_organization) { organizations.create!(id: 1, name: 'default', path: 'default') }
+  let(:organization) { organizations.create!(name: 'organization', path: 'organization') }
+  let(:namespace1) { namespaces.create!(name: 'namespace 1', path: 'namespace1', organization_id: organization.id) }
+  let(:project1) do
+    projects.create!(namespace_id: namespace1.id, project_namespace_id: namespace1.id, organization_id: organization.id)
+  end
+
+  let(:issue1) do
+    issues.create!(namespace_id: namespace1.id, project_id: project1.id, created_at: 5.days.ago, closed_at: 3.days.ago,
+      work_item_type_id: work_item_type_id)
+  end
+
+  let(:issue2) do
+    issues.create!(namespace_id: namespace1.id, project_id: project1.id, created_at: 7.days.ago, closed_at: 2.days.ago,
+      work_item_type_id: work_item_type_id)
+  end
+
+  let(:work_item_type_id) { table(:work_item_types).where(base_type: 1).first.id }
+
+  describe '#perform' do
+    subject(:migration) do
+      described_class.new(
+        start_id: user_agent_details.minimum(:id),
+        end_id: user_agent_details.maximum(:id),
+        batch_table: :user_agent_details,
+        batch_column: :id,
+        sub_batch_size: 100,
+        pause_ms: 0,
+        connection: connection
+      )
+    end
+
+    before do
+      create_user_agent_detail!(organization: organization, target: issue1)
+      create_user_agent_detail!(organization: organization, target: issue2)
+      create_user_agent_detail!(organization: nil, target: issue1)
+      create_user_agent_detail!(organization: nil, target: issue2)
+    end
+
+    it 'updates records without an organization' do
+      expect { migration.perform }.to change { user_agent_details.where(organization_id: nil).count }.from(2).to(0)
+    end
+  end
+
+  def create_user_agent_detail!(target:, organization: nil)
+    user_agent_details.create!(
+      organization_id: organization&.id,
+      ip_address: '127.0.0.1',
+      user_agent: 'Mozilla/5.0 (X11; Linux i686; rv:136.0) Gecko/20100101 Firefox/136.0',
+      subject_id: target&.id,
+      subject_type: target.class.name
+    )
+  end
+end
diff --git a/spec/migrations/20251215170942_queue_backfill_user_agent_details_organization_id_spec.rb b/spec/migrations/20251215170942_queue_backfill_user_agent_details_organization_id_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..f693440b021e3cd9c5093866eeabae14e80e23eb
--- /dev/null
+++ b/spec/migrations/20251215170942_queue_backfill_user_agent_details_organization_id_spec.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!
+
+RSpec.describe QueueBackfillUserAgentDetailsOrganizationId, migration: :gitlab_main_org, feature_category: :instance_resiliency 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(
+          gitlab_schema: :gitlab_main_org,
+          table_name: :user_agent_details,
+          column_name: :id,
+          batch_size: described_class::BATCH_SIZE,
+          sub_batch_size: described_class::SUB_BATCH_SIZE
+        )
+      }
+    end
+  end
+end