Seym (also transcribed as Seim and Sejm, in Russian: Сейм; Ukrainian: Сейм) is a west-flowing river in Russia and Ukraine. Its length is 748 kilometres (465 mi) (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.
Seym in Kursk Oblast
| ⁃ location||Russia|
|Length||748 km (465 mi)|
|Basin size||27,500 km2 (10,600 sq mi)|
| ⁃ average||99,6 m³/s|
Places on the river are: Kursk, Kurchatov, Rylsk, Ukrainian border, Putyvl, Baturin, junction with the Desna River which continues west and south past Chernogov to Kiev.