diff --git a/migrations/000.268.sql b/migrations/000.268.sql
new file mode 100644
index 0000000..393e182
--- /dev/null
+++ b/migrations/000.268.sql
@@ -0,0 +1,3 @@
+-- P5-6290: Add unique constraint to market_locales
+
+ALTER TABLE market_locales ADD UNIQUE (market_id, locale_id);