Gmina Rząśnik is a rural gmina (administrative district) in Wyszków County, Masovian Voivodeship, in east-central Poland. Its seat is the village of Rząśnik, which lies approximately 14 kilometres (9 mi) north-west of Wyszków and 60 km (37 mi) north-east of Warsaw.
The gmina covers an area of 167.42 square kilometres (64.6 sq mi), and as of 2006 its total population is 6,603 (6,965 in 2013).