The Seym or Seim is a river that flows westward in Russia and Ukraine. It is 748 kilometres long and its basin area about 27,500 square kilometres. 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.