diff --git a/db/post_migrate/20250926213835_backfill_design_management_designs_sync.rb b/db/post_migrate/20250926213835_backfill_design_management_designs_sync.rb
new file mode 100644
index 0000000000000000000000000000000000000000..731ecebfc718dae4c1298f7b6d57e2f719c65adb
--- /dev/null
+++ b/db/post_migrate/20250926213835_backfill_design_management_designs_sync.rb
@@ -0,0 +1,65 @@
+# frozen_string_literal: true
+
+class BackfillDesignManagementDesignsSync < Gitlab::Database::Migration[2.3]
+  disable_ddl_transaction!
+  milestone '18.5'
+  restrict_gitlab_migration gitlab_schema: :gitlab_main
+
+  def up
+    each_batch(:design_management_designs, of: 100) do |sub_batch|
+      sub_batch_ids = sub_batch.select(:id)
+      update_design_management_designs_namespace_id!(sub_batch_ids)
+
+      # Since the design_management_designs table now has the correct namespace_id, we can now update the values on
+      # the child tables (design_management_designs_versions and design_user_mentions)
+      update_design_management_designs_versions_namespace_id!(sub_batch_ids)
+      update_design_user_mentions_namespace_id!(sub_batch_ids)
+    end
+  end
+
+  def down
+    # no-op
+  end
+
+  private
+
+  # Sets the namespace_id on design_management_designs to the corresponding project's project_namespace_id
+  def update_design_management_designs_namespace_id!(sub_batch_ids)
+    sql = <<~SQL
+      UPDATE design_management_designs
+      SET namespace_id = p.project_namespace_id
+      FROM projects p
+      WHERE design_management_designs.project_id = p.id
+      AND design_management_designs.id IN (#{sub_batch_ids.to_sql})
+    SQL
+
+    ApplicationRecord.connection.execute(sql)
+  end
+
+  # Sets the namespace_id on design_management_designs_versions to the corresponding design_management_designs's
+  # namespace_id
+  def update_design_management_designs_versions_namespace_id!(sub_batch_ids)
+    sql = <<~SQL
+      UPDATE design_management_designs_versions
+      SET namespace_id = design_management_designs.namespace_id
+      FROM design_management_designs
+      WHERE design_management_designs.id = design_management_designs_versions.design_id
+      AND design_management_designs_versions.design_id IN (#{sub_batch_ids.to_sql})
+    SQL
+
+    ApplicationRecord.connection.execute(sql)
+  end
+
+  # Sets the namespace_id on design_user_mentions to the corresponding design_management_designs's namespace_id
+  def update_design_user_mentions_namespace_id!(sub_batch_ids)
+    sql = <<~SQL
+      UPDATE design_user_mentions
+      SET namespace_id = design_management_designs.namespace_id
+      FROM design_management_designs
+      WHERE design_management_designs.id = design_user_mentions.design_id
+      AND design_user_mentions.design_id IN (#{sub_batch_ids.to_sql})
+    SQL
+
+    ApplicationRecord.connection.execute(sql)
+  end
+end
diff --git a/db/schema_migrations/20250926213835 b/db/schema_migrations/20250926213835
new file mode 100644
index 0000000000000000000000000000000000000000..2ae23c752665903f068b6e08245a507b3bf521e2
--- /dev/null
+++ b/db/schema_migrations/20250926213835
@@ -0,0 +1 @@
+fa364993482b54ccc6c77f681e5849cfcafa78dae20affc3f68826e7a00f2f0e
\ No newline at end of file