Legionowo County (Polish: powiat legionowski) is a unit of territorial administration and local government (powiat) in Masovian Voivodeship, east-central Poland. It came into being on January 1, 1999, as a result of the Polish local government reforms passed in 1998. Its administrative seat and largest town is Legionowo, which lies 22 kilometres (14 mi) north of Warsaw. The only other town in the county is Serock, lying 17 km (11 mi) north-east of Legionowo.

The county covers an area of 389.86 square kilometres (150.5 sq mi). As of 2006 its total population is 96,497, out of which the population of Legionowo is 50,698, that of Serock is 3,721, and the rural population is 42,078.

Legionowo is part of the greater Warsaw area and has a large commuter population. The train connection takes about 30 mins to central Warsaw (connection to Warsaw Metro at Warszawa Gdańska station) and is planned to be added to the Warsaw Rapid Train system by 2010. Both the road and the train connection pass through the national reserve forest which separates Legionowo from Warsaw and is a favorite weekend walk and biking destination. On the opposite side of Legionowo is the Zegrze Reservoir, created in 1970 with the building of a river dam, now a popular leisure destination.

Legionowo [lɛɡʲɔˈnɔvɔ] ( listen) is a town in Masovia (Polish: Mazowsze). According to the 2004 Census estimate the town has a total population of 50,759.

Legionowo is located ca. 23 km to the north-east of the center of Warsaw and only 7 km to the south of Zegrze Reservoir (Jezioro Zegrzyńskie or Zalew Zegrzyński), near the Warsaw-Gdańsk railroad and Warsaw-Suwałki road. City area is 1360ha (13.6 km²), which makes population density 3727.79 persons/km². Altitude: 75–85 m above sea level.

Situated in the Masovian Voivodeship (województwo mazowieckie, created in 1999 as a result of Local Government Reorganization Act), previously in Warszawa Voivodeship (województwo warszawskie or województwo stołeczne warszawskie, 1975–1998) and old Masovian Voivodeship (before 1975). Currently this is the capital of Legionowo County (powiat legionowski), which is one of 38 land counties (powiat ziemski) in Masovian Voivodeship.

Adjoining counties (from north, clockwise): Pułtusk County, Wyszków County, Wołomin County, Warszawa County, Warszawa Zachód County, Nowy Dwór Mazowiecki County.