The Seym or Seim (Russian: Сейм; Ukrainian: Сейм) is a river that flows westward in Russia and Ukraine. It is 748 kilometres (465 mi) long (250 km within Ukraine) and its basin area about 27,500 square kilometres (10,600 sq mi). It is the largest tributary of the Desna.
Places on the river include Kursk, Kurchatov, Rylsk, Glushkovo, Putyvl, Baturyn, and the junction with the Desna, which continues west and south past Chernihiv to Kyiv.