The Seym or Seim is a river that flows westward in Russia and Ukraine. It is 748 km long (250 km within Ukraine) and its basin area about 27500 km2. 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. In September 2024, during the Russian invasion of Ukraine, Ukrainian officials accused Russia of polluting the river by dumping sewage from a sugar factory in Tyotkino, Kursk Oblast, resulting in environmental damages estimated at about Hr 186 million ($4.5 million) in Chernihiv Oblast that included a decrease in oxygen levels and fish kills downstream.