Nahr as-Sinn () is a river in Syria which marks the borders between Tartus Governorate and Latakia Governorate.
In ancient times, the river was called "Siyannu" which marked the southern borders of Ugarit.
Nahr as-Sinn rises in a plain area in Tartus Governorate and runs to the west to finally flow into the Mediterranean Sea south of Arab al-Mulk. Despite its length which is only , it is considered the richest river in Syria.