Sejny is a town located in north-eastern Poland and serves as the capital of Sejny County within the Podlaskie Voivodeship. It is situated in close proximity to the national borders of Lithuania and Belarus.
Geographically, the town is positioned in the eastern part of the Suwałki Lake Area along the Marycha river, which is a tributary of the Czarna Hańcza. As of 1999, Sejny had a permanent population of nearly 6,500 residents, though this number increases significantly during the tourist season.