(Russian, Shiashupe; Polish, Szeszupa), a river that flows in the Lithuanian SSR and Kaliningrad Oblast, RSFSR, and has its source in Poland. A left tributary of the Neman, the Šešupė is 298 km long and drains an area of 6,100 sq km. The river, which rises in the Baltic Ridge, is fed primarily by rain. The mean flow rate 43 km from the mouth is 33.2 cu m per sec. The river freezes over between mid-November and January, and the ice breaks up between late February and early April. Cities on the Šešupė include Kalvarija, Kapsukas, Kudirkos Naumiestis, and Krasnoznamensk.