Gmina Leoncin is a rural gmina (administrative district) in Nowy Dwór Mazowiecki County, Masovian Voivodeship, in east-central Poland. Its seat is the village of Leoncin, which lies approximately 12 kilometres (7 mi) south-west of Nowy Dwór Mazowiecki and 37 km (23 mi) north-west of Warsaw.The gmina covers an area of 158.84 square kilometres (61.3 sq mi), and as of 2006 its total population is 5,092 (5,435 in 2011).