Sejny County (Polish: powiat sejneński) is a unit of territorial administration and local government (powiat) in Podlaskie Voivodeship, in the extreme north-east of Poland, on the border with Lithuania and Belarus. It was created on January 1, 1999 as a result of the Local Government Reorganization Act of 1998. Its administrative seat and only town is Sejny, which lies 112 kilometres (70 mi) north of the regional capital Białystok.
The county covers an area of 856.07 square kilometres (330.5 sq mi). As of 2006 its total population is 21,331, out of which the population of Sejny is 5,934 and the rural population is 15,397.