Sejny County 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 km north of the regional capital Białystok. The county covers an area of 856.07 km2. As of 2019 its total population was 20,002, with Lithuanians comprising 20.2% (4,271) of the inhabitants.

Neighbouring counties

Sejny County is bordered by Augustów County to the south and Suwałki County to the west. It also borders Lithuania to the north-east and Belarus to the east.

Administrative division

The county is subdivided into five gminas (one urban and four rural). These are listed in the following table, in descending order of population.