Sejny County (Polish: powiat sejneński) 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 came into being on January 1, 1999, as a result of the Polish local government reforms passed in 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).

The county is subdivided into five gminas (one urban and four rural).