Gmina Miastkowo is a rural gmina (administrative district) in Łomża County, Podlaskie Voivodeship, in north-eastern Poland. Its seat is the village of Miastkowo, which lies approximately 18 kilometres (11 mi) west of Łomża and 91 km (57 mi) west of the regional capital Białystok.
The gmina covers an area of 114.84 square kilometres (44.3 sq mi), and as of 2006 its total population is 4,306 (4,350 in 2011).