Ostrołęka County (Polish: powiat ostrołęcki) is a unit of territorial administration and local government (powiat) in Masovian Voivodeship, east-central Poland. It was created on January 1, 1999 as a result of the Local Government Reorganization Act of 1998. Its administrative seat is the city of Ostrołęka, although the city is not part of the county (it constitutes a separate city county). The only town in Ostrołęka County is Myszyniec, which lies 38 km (24 mi) north of Ostrołęka.
The county covers an area of 2,099.32 square kilometres (810.6 sq mi). As of 2006 its total population is 84,344, out of which the population of Myszyniec is 3,014 and the rural population is 81,330.