diff --git a/db/post_migrate/20251016010313_add_missing_merge_requests_indexes_for_bigint_swap.rb b/db/post_migrate/20251016010313_add_missing_merge_requests_indexes_for_bigint_swap.rb
new file mode 100644
index 0000000000000000000000000000000000000000..8a7e688174ab0c7a83584b888995a59a79e15a1f
--- /dev/null
+++ b/db/post_migrate/20251016010313_add_missing_merge_requests_indexes_for_bigint_swap.rb
@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+class AddMissingMergeRequestsIndexesForBigintSwap < Gitlab::Database::Migration[2.3]
+  milestone '18.6'
+  disable_ddl_transaction!
+
+  TABLE_NAME = 'merge_requests'
+  INDEXES = [
+    {
+      name: 'index_merge_requests_on_assignee_id',
+      columns: [:assignee_id]
+    },
+    {
+      name: 'index_merge_requests_on_merge_user_id',
+      columns: [:merge_user_id],
+      options: { where: "merge_user_id IS NOT NULL" }
+    },
+    {
+      name: 'index_merge_requests_on_updated_by_id',
+      columns: [:updated_by_id],
+      options: { where: "updated_by_id IS NOT NULL" }
+    },
+    {
+      name: 'index_merge_requests_on_milestone_id',
+      columns: [:milestone_id]
+    },
+    {
+      name: 'idx_merge_requests_on_source_project_and_branch_state_opened',
+      columns: [:source_project_id, :source_branch],
+      options: { where: "state_id = 1" }
+    },
+    {
+      name: 'index_merge_requests_on_source_project_id_and_source_branch',
+      columns: [:source_project_id, :source_branch]
+    }
+  ].freeze
+
+  def up
+    # rubocop:disable Migration/PreventIndexCreation -- add existing indexes for bigint migration
+    INDEXES.each do |index|
+      options = index[:options] || {}
+      add_concurrent_index TABLE_NAME, index[:columns], name: index[:name], if_not_exists: true, **options
+    end
+    # rubocop:enable Migration/PreventIndexCreation
+  end
+
+  def down
+    # no-op
+  end
+end
diff --git a/db/schema_migrations/20251016010313 b/db/schema_migrations/20251016010313
new file mode 100644
index 0000000000000000000000000000000000000000..8c6bacfa8e32bca5ec08399765544eaf2f0ae5d7
--- /dev/null
+++ b/db/schema_migrations/20251016010313
@@ -0,0 +1 @@
+17cbdf721008065d0d03032cb0574f5df66e1561b96ca7ccff64780020ac60ca
\ No newline at end of file