diff --git a/database/migrations/2026_07_02_133321_add_user_source_created_at_index_to_transactions_table.php b/database/migrations/2026_07_02_133321_add_user_source_created_at_index_to_transactions_table.php
new file mode 100644
index 00000000..373f5c9e
--- /dev/null
+++ b/database/migrations/2026_07_02_133321_add_user_source_created_at_index_to_transactions_table.php
@@ -0,0 +1,30 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Support the daily "transactions synced" email query, which filters
+     * transactions by (user_id = ?, source = ?, created_at > ?). The existing
+     * indexes only lead with user_id via transaction_date, so that query scans
+     * every one of the user's rows to filter on source + created_at
+     * (PHP-LARAVEL-3X). This composite index matches the two equalities and the
+     * range directly.
+     */
+    public function up(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            $table->index(['user_id', 'source', 'created_at'], 'idx_transactions_user_source_created');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('transactions', function (Blueprint $table) {
+            $table->dropIndex('idx_transactions_user_source_created');
+        });
+    }
+};
diff --git a/tests/Feature/OpenBanking/DailyEmailTransactionsIndexTest.php b/tests/Feature/OpenBanking/DailyEmailTransactionsIndexTest.php
new file mode 100644
index 00000000..03c3b021
--- /dev/null
+++ b/tests/Feature/OpenBanking/DailyEmailTransactionsIndexTest.php
@@ -0,0 +1,14 @@
+<?php
+
+use Illuminate\Support\Facades\Schema;
+
+it('has a composite index supporting the daily synced-email transaction query', function () {
+    $indexes = collect(Schema::getIndexes('transactions'));
+
+    $match = $indexes->first(fn (array $index): bool => $index['columns'] === ['user_id', 'source', 'created_at']
+    );
+
+    expect($match)->not->toBeNull(
+        'Expected a (user_id, source, created_at) index on transactions for the daily email query.'
+    );
+});