Gmina Piątnica is a rural gmina (administrative district) in Łomża County, Podlaskie Voivodeship, in north-eastern Poland. Its seat is the village of Piątnica, which lies approximately 3 kilometres (2 mi) north-east of Łomża and 72 km (45 mi) west of the regional capital Białystok.
The gmina covers an area of 218.69 square kilometres (84.4 sq mi), and as of 2006 its total population is 10,642 (10,787 in 2011).