Insar (, ) 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<sup>2</sup>. 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<sup>3</sup>/s. The river freezes over in November, and is icebound until April.
Towns by the Insar are Ruzayevka and Saransk.