diff --git a/db/post_migrate/20250820035405_prepare_assignee_id_indexes_for_merge_requests_bigint_conversion.rb b/db/post_migrate/20250820035405_prepare_assignee_id_indexes_for_merge_requests_bigint_conversion.rb
new file mode 100644
index 0000000000000000000000000000000000000000..9c7454a152bf2e5e87508fe2dfefdb365f052f47
--- /dev/null
+++ b/db/post_migrate/20250820035405_prepare_assignee_id_indexes_for_merge_requests_bigint_conversion.rb
@@ -0,0 +1,82 @@
+# frozen_string_literal: true
+
+class PrepareAssigneeIdIndexesForMergeRequestsBigintConversion < Gitlab::Database::Migration[2.3]
+  include Gitlab::Database::MigrationHelpers::ConvertToBigint
+
+  disable_ddl_transaction!
+  milestone '18.4'
+
+  TABLE_NAME = 'merge_requests'
+  BIGINT_COLUMNS = [
+    :assignee_id_convert_to_bigint,
+    :merge_user_id_convert_to_bigint,
+    :updated_by_id_convert_to_bigint,
+    :milestone_id_convert_to_bigint,
+    :source_project_id_convert_to_bigint
+  ].freeze
+
+  INDEXES = [
+    {
+      name: 'index_merge_requests_on_assignee_id',
+      columns: [:assignee_id_convert_to_bigint]
+    },
+    {
+      name: 'index_merge_requests_on_merge_user_id',
+      columns: [:merge_user_id_convert_to_bigint],
+      options: { where: "merge_user_id_convert_to_bigint IS NOT NULL" }
+    },
+    {
+      name: 'index_merge_requests_on_updated_by_id',
+      columns: [:updated_by_id_convert_to_bigint],
+      options: { where: "updated_by_id_convert_to_bigint IS NOT NULL" }
+    },
+    {
+      name: 'index_merge_requests_on_milestone_id',
+      columns: [:milestone_id_convert_to_bigint]
+    },
+    {
+      name: 'idx_merge_requests_on_source_project_and_branch_state_opened',
+      columns: [:source_project_id_convert_to_bigint, :source_branch],
+      options: { where: "state_id = 1" }
+    },
+    {
+      name: 'index_merge_requests_on_source_project_id_and_source_branch',
+      columns: [:source_project_id_convert_to_bigint, :source_branch]
+    }
+  ].freeze
+
+  def up
+    return if skip_migration?
+
+    # rubocop:disable Migration/PreventIndexCreation -- Bigint migration
+    INDEXES.each do |index|
+      options = index[:options] || {}
+      prepare_async_index(TABLE_NAME, index[:columns], name: bigint_index_name(index[:name]), **options)
+    end
+    # rubocop:enable Migration/PreventIndexCreation
+  end
+
+  def down
+    return if skip_migration?
+
+    INDEXES.each do |index|
+      options = index[:options] || {}
+      unprepare_async_index(TABLE_NAME, index[:columns], name: bigint_index_name(index[:name]), **options)
+    end
+  end
+
+  private
+
+  def skip_migration?
+    unless conversion_columns_exist?
+      say "No conversion columns found - migration skipped"
+      return true
+    end
+
+    false
+  end
+
+  def conversion_columns_exist?
+    BIGINT_COLUMNS.all? { |column| column_exists?(TABLE_NAME, column) }
+  end
+end
diff --git a/db/schema_migrations/20250820035405 b/db/schema_migrations/20250820035405
new file mode 100644
index 0000000000000000000000000000000000000000..48f77f95b033a8d0d8a725607fa159e7fbbbebd8
--- /dev/null
+++ b/db/schema_migrations/20250820035405
@@ -0,0 +1 @@
+7bc886716ae3670543764248e7f5d0fb67ebac2a5907ca2133a88f29e7c6ea7f
\ No newline at end of file