The Šešupė (pronounced [ˈʃɛːʃʊːpeː] ); Russian: Шешупе; Polish: Szeszupa; German: Scheschup(p)e, Ostfluss) is a 298 km long river that flows through Poland (27 km), Lithuania (158 km), and Russia (62 km). The river flows for 51 km along the border between the Kaliningrad Oblast, an exclave of Russia, and Lithuania. The Šešupė originates near the Polish village of Szeszupka, about 16 km from the Polish-Lithuanian border, and flows into the Nemunas, near the town of Neman, on the border between Lithuania and Kaliningrad Oblast.
Major towns and cities along the river, from the Nemunas to the source, are: Kudirkos Naumiestis, Pilviškiai, Marijampolė, and Kalvarija.