diff --git a/db/docs/batched_background_migrations/backfill_namespace_details_description_fields.yml b/db/docs/batched_background_migrations/backfill_namespace_details_description_fields.yml
new file mode 100644
index 0000000000000000000000000000000000000000..936d52bf665fc3a061240ced27498bae5635215e
--- /dev/null
+++ b/db/docs/batched_background_migrations/backfill_namespace_details_description_fields.yml
@@ -0,0 +1,8 @@
+---
+migration_job_name: BackfillNamespaceDetailsDescriptionFields
+description: backfill the namespace_details table with the description, description_html and cached_markdown_version
+feature_category: groups_and_projects
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/209801
+milestone: '18.6'
+queued_migration_version: 20251022120652
+finalized_by: # version of the migration that finalized this BBM
diff --git a/db/post_migrate/20251022120652_queue_backfill_namespace_details_description_fields.rb b/db/post_migrate/20251022120652_queue_backfill_namespace_details_description_fields.rb
new file mode 100644
index 0000000000000000000000000000000000000000..1fa5de7fa834a25d0435340cf69c7c10b4e2c28a
--- /dev/null
+++ b/db/post_migrate/20251022120652_queue_backfill_namespace_details_description_fields.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+class QueueBackfillNamespaceDetailsDescriptionFields < Gitlab::Database::Migration[2.3]
+  milestone '18.6'
+  restrict_gitlab_migration gitlab_schema: :gitlab_main
+
+  MIGRATION = "BackfillNamespaceDetailsDescriptionFields"
+  DELAY_INTERVAL = 2.minutes
+  BATCH_SIZE = 1000
+  SUB_BATCH_SIZE = 100
+
+  def up
+    # Check if the migration already exists to avoid re-queueing
+    if Gitlab::Database::BackgroundMigration::BatchedMigration.for_configuration(
+      :gitlab_main,
+      MIGRATION,
+      :namespace_details,
+      :namespace_id,
+      [],
+      include_compatible: true
+    ).exists?
+      Gitlab::AppLogger.warn "Batched background migration not enqueued because it already exists: " \
+        "job_class_name: BackfillNamespaceDetailsDescriptionFields,
+        table_name: namespace_details, column_name: namespace_id, " \
+        "job_arguments: []"
+      return
+    end
+
+    queue_batched_background_migration(
+      MIGRATION,
+      :namespace_details,
+      :namespace_id,
+      job_interval: DELAY_INTERVAL,
+      batch_size: BATCH_SIZE,
+      sub_batch_size: SUB_BATCH_SIZE
+    )
+  end
+
+  def down
+    delete_batched_background_migration(MIGRATION, :namespace_details, :namespace_id, [])
+  end
+end
diff --git a/db/schema_migrations/20251022120652 b/db/schema_migrations/20251022120652
new file mode 100644
index 0000000000000000000000000000000000000000..2bdb306471113f16e49f5faf42fbe8bb636c0dc3
--- /dev/null
+++ b/db/schema_migrations/20251022120652
@@ -0,0 +1 @@
+a4cabdf794253708135b9fdc00116c3a1098afeadaeddf50c9dab76ddd89de05
\ No newline at end of file
diff --git a/lib/gitlab/background_migration/backfill_namespace_details_description_fields.rb b/lib/gitlab/background_migration/backfill_namespace_details_description_fields.rb
new file mode 100644
index 0000000000000000000000000000000000000000..49c954579430ff1f9d472cf4ac36f98cc7919f9a
--- /dev/null
+++ b/lib/gitlab/background_migration/backfill_namespace_details_description_fields.rb
@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module BackgroundMigration
+    class BackfillNamespaceDetailsDescriptionFields < BatchedMigrationJob
+      operation_name :backfill_namespace_details_description_fields
+      feature_category :groups_and_projects
+
+      def perform
+        each_sub_batch do |sub_batch|
+          backfill_namespace_details_fields(sub_batch)
+        end
+      end
+
+      private
+
+      def backfill_namespace_details_fields(relation)
+        connection.execute(<<~SQL)
+          UPDATE namespace_details
+          SET description = CASE WHEN namespace_details.description IS NULL THEN namespaces.description ELSE namespace_details.description END,
+              description_html = CASE WHEN namespace_details.description_html IS NULL THEN namespaces.description_html ELSE namespace_details.description_html END,
+              cached_markdown_version = CASE WHEN namespace_details.cached_markdown_version IS NULL THEN namespaces.cached_markdown_version ELSE namespace_details.cached_markdown_version END
+          FROM namespaces
+          WHERE namespace_details.namespace_id = namespaces.id
+            AND namespace_details.namespace_id IN (#{relation.select(:namespace_id).to_sql})
+            AND (
+              (namespace_details.description IS NULL AND namespaces.description IS NOT NULL)
+              OR (namespace_details.description_html IS NULL AND namespaces.description_html IS NOT NULL)
+              OR (namespace_details.cached_markdown_version IS NULL AND namespaces.cached_markdown_version IS NOT NULL)
+            )
+        SQL
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/background_migration/backfill_namespace_details_description_fields_spec.rb b/spec/lib/gitlab/background_migration/backfill_namespace_details_description_fields_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..f26fe7e2f9f5a2f84df0af2c63fbd5812a2332b3
--- /dev/null
+++ b/spec/lib/gitlab/background_migration/backfill_namespace_details_description_fields_spec.rb
@@ -0,0 +1,180 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::BackgroundMigration::BackfillNamespaceDetailsDescriptionFields, feature_category: :groups_and_projects do
+  let(:organizations) { table(:organizations) }
+  let(:namespaces) { table(:namespaces) }
+  let(:namespace_details) { table(:namespace_details) }
+  let!(:organization) { organizations.create!(name: 'Test Org', path: 'test-org') }
+
+  subject(:perform_migration) do
+    described_class.new(
+      start_id: namespace_details.minimum(:namespace_id),
+      end_id: namespace_details.maximum(:namespace_id),
+      batch_table: :namespace_details,
+      batch_column: :namespace_id,
+      sub_batch_size: 10,
+      pause_ms: 0,
+      connection: ApplicationRecord.connection
+    ).perform
+  end
+
+  describe '#perform' do
+    context 'when namespace has description fields and namespace_details do not' do
+      # these columns are added to ignore_columns :description, :description_html, :cached_markdown_version
+      # need to manually populate these fields
+      let!(:namespace1) do
+        namespaces.create!(
+          name: 'namespace-1',
+          path: 'namespace-1',
+          type: 'Group',
+          organization_id: organization.id
+        )
+      end
+
+      let!(:namespace_detail1) do
+        namespace_details.insert({
+          namespace_id: namespace1.id,
+          description: nil,
+          description_html: nil,
+          cached_markdown_version: nil,
+          created_at: Time.current,
+          updated_at: Time.current
+        })
+
+        ApplicationRecord.connection.execute(<<~SQL)
+          UPDATE namespaces
+          SET description = 'Test description 1',
+              description_html = '<p>Test description 1</p>',
+              cached_markdown_version = 1
+          WHERE id = #{namespace1.id}
+        SQL
+        namespace_details.find(namespace1.id)
+      end
+
+      let!(:namespace2) do
+        namespaces.create!(
+          name: 'namespace-2',
+          path: 'namespace-2',
+          type: 'Group',
+          organization_id: organization.id
+        )
+      end
+
+      let!(:namespace_detail2) do
+        namespace_details.insert({
+          namespace_id: namespace2.id,
+          description: nil,
+          description_html: nil,
+          cached_markdown_version: nil,
+          created_at: Time.current,
+          updated_at: Time.current
+        })
+        ApplicationRecord.connection.execute(<<~SQL)
+          UPDATE namespaces
+          SET description = 'Test description 2',
+              description_html = '<p>Test description 2</p>',
+              cached_markdown_version = 3
+          WHERE id = #{namespace2.id}
+        SQL
+        namespace_details.find(namespace2.id)
+      end
+
+      it 'backfills namespace_details with description fields from namespaces' do
+        perform_migration
+
+        detail1 = namespace_details.find(namespace1.id)
+        detail2 = namespace_details.find(namespace2.id)
+
+        expect(detail1.description).to eq('Test description 1')
+        expect(detail1.description_html).to eq('<p>Test description 1</p>')
+        expect(detail1.cached_markdown_version).to eq(1)
+
+        expect(detail2.description).to eq('Test description 2')
+        expect(detail2.description_html).to eq('<p>Test description 2</p>')
+        expect(detail2.cached_markdown_version).to eq(3)
+      end
+    end
+
+    context 'when namespace_details already have description fields' do
+      let!(:namespace) do
+        namespaces.create!(
+          name: 'namespace-existing',
+          path: 'namespace-existing',
+          type: 'Group',
+          organization_id: organization.id
+        )
+      end
+
+      let!(:namespace_detail) do
+        ApplicationRecord.connection.execute(<<~SQL)
+          UPDATE namespaces
+          SET description = 'New description',
+              description_html = '<p>New description</p>',
+              cached_markdown_version = 999999
+          WHERE id = #{namespace.id}
+        SQL
+
+        ApplicationRecord.connection.execute(<<~SQL)
+          UPDATE namespace_details
+          SET description = 'Existing description',
+              description_html = '<p>Existing description</p>',
+              cached_markdown_version = 111111
+          WHERE namespace_id = #{namespace.id}
+        SQL
+        namespace_details.find(namespace.id)
+      end
+
+      it 'does not overwrite existing namespace_details fields' do
+        perform_migration
+
+        detail = namespace_details.find(namespace.id)
+
+        expect(detail.description).to eq('Existing description')
+        expect(detail.description_html).to eq('<p>Existing description</p>')
+        expect(detail.cached_markdown_version).to eq(111111)
+      end
+    end
+
+    context 'when namespace_details has partial fields' do
+      let!(:namespace) do
+        namespaces.create!(
+          name: 'namespace-partial-details',
+          path: 'namespace-partial-details',
+          type: 'Group',
+          organization_id: organization.id
+        )
+      end
+
+      let!(:namespace_detail) do
+        ApplicationRecord.connection.execute(<<~SQL)
+          UPDATE namespaces
+          SET description = 'Updated description',
+              description_html = '<p>Updated description</p>',
+              cached_markdown_version = 777777
+          WHERE id = #{namespace.id}
+        SQL
+
+        ApplicationRecord.connection.execute(<<~SQL)
+          UPDATE namespace_details
+          SET description = 'Existing description',
+              description_html = NULL,
+              cached_markdown_version = NULL
+          WHERE namespace_id = #{namespace.id}
+        SQL
+        namespace_details.find(namespace.id)
+      end
+
+      it 'only backfills the null fields' do
+        perform_migration
+
+        detail = namespace_details.find(namespace.id)
+
+        expect(detail.description).to eq('Existing description')
+        expect(detail.description_html).to eq('<p>Updated description</p>')
+        expect(detail.cached_markdown_version).to eq(777777)
+      end
+    end
+  end
+end
diff --git a/spec/migrations/20251022120652_queue_backfill_namespace_details_description_fields_spec.rb b/spec/migrations/20251022120652_queue_backfill_namespace_details_description_fields_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..e8550de53f573000d03374994d67e3275fd4cf49
--- /dev/null
+++ b/spec/migrations/20251022120652_queue_backfill_namespace_details_description_fields_spec.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+require_migration!
+
+RSpec.describe QueueBackfillNamespaceDetailsDescriptionFields, migration: :gitlab_main, feature_category: :groups_and_projects 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: :namespace_details,
+          column_name: :namespace_id,
+          interval: described_class::DELAY_INTERVAL,
+          batch_size: described_class::BATCH_SIZE,
+          sub_batch_size: described_class::SUB_BATCH_SIZE,
+          gitlab_schema: :gitlab_main,
+          job_arguments: []
+        )
+      }
+    end
+  end
+end