Its administrative seat and largest town is Bielsk Podlaski, which lies 39 kilometres (24 mi) south of the regional capital Białystok.

On 18 January 1493, Brańsk received a city charter based on Magdeburg rights from the Grand Duke of Lithuania, Alexander Jagiellon.

On 18 November 1495, Bielsk Podlaski received a city charter based on Magdeburg rights from the Grand Duke of Lithuania, Alexander Jagiellon.

On 1 July 1569, the Union of Lublin was signed between the Grand Duchy of Lithuania and the Kingdom of Poland forming the Polish–Lithuanian Commonwealth.

The treaties repartitioned former Polish territory and assigned Bielsk County and most of eastern Podlaskie from Prussia to Imperial Russia.