Gmina Sienno is a rural gmina (administrative district) in Lipsko County, Masovian Voivodeship, in east-central Poland. Its seat is the village of Sienno, which lies approximately 15 kilometres (9 mi) south-west of Lipsko and 130 km (81 mi) south of Warsaw.
The gmina covers an area of 147.15 square kilometres (56.8 sq mi), and as of 2006 its total population is 6,377.