Ostrów County (Polish: powiat ostrowski) is a county in Masovian Voivodeship, east-central Poland.

The county was established on 1 January 1999, and historically existed from 1919 to 1939 in the Białystok Voivodeship, Second Polish Republic, and in 1939, in the Warsaw Voivodeship.

The county covers an area of 1,218.06 square kilometres (470.3 sq mi).

The county is subdivided into 11 gminas (one urban, one urban-rural and nine rural).

These are listed in the following table, in descending order of population.