Nowy Dwór Mazowiecki County (Polish: powiat nowodworski) 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 and largest town is Nowy Dwór Mazowiecki, which lies 33 kilometres (21 mi) north-west of Warsaw. The county also contains the towns of Nasielsk, lying 21 km (13 mi) north-east of Nowy Dwór Mazowiecki, and Zakroczym, 5 km (3 mi) west of Nowy Dwór Mazowiecki.
The county covers an area of 691.65 square kilometres (267.0 sq mi). As of 2006 its total population is 75,736, out of which the population of Nowy Dwór Mazowiecki is 27,545, that of Nasielsk is 7,364, that of Zakroczym is 3,367, and the rural population is 37,460.
The county is subdivided into six gminas (one urban, two urban-rural and three rural). These are listed in the following table, in descending order of population.