The Sit' River (Сить) is a tributary of the Rybinsk Reservoir (nominally, of the Mologa River). The river flows for through Sonkovsky District of Tver Oblast and Nekouzsky and Breytovsky Districts of Yaroslavl Oblasts of Russia before entering the Rybinsk Reservoir near the large village of Breitovo. Its average width varies from . The river mouth is about wide. The drainage basin occupies some . The river is long.