Insar (Russian: Инса́р, Erzya: Инесаро) is a river in the Mordovian republic of Russia. It is a right tributary of the Alatyr. It is 168 km long, with a drainage basin of 3860 km. The river gets most of its water from melting snow, and hence its discharge is at its highest during the spring thaw. Average discharge at Saransk is 7.71 m/s. The river freezes over in November, and is icebound until April.

Towns by the Insar are Ruzayevka and Saransk.