Rollag is a municipality in Buskerud county, Norway. It is part of the traditional region of Numedal. The municipality of Rollag was established on 1 January 1838 (see formannskapsdistrikt). Nore og Uvdal was separated from Rollag in 1858.
The administrative centre of the municipality is the village of Rollag, although the most populated area in the municipality is Veggli. Rollag is bordered in the north by Nore og Uvdal, in the east by Sigdal, in the south by Flesberg, and in the west by Tinn (Telemark county).
The municipality (originally the parish) is named after the old Rollag farm (Old Norse: Roll(u)lag), since the first church was built here. The first element is probably (the genitive case of) a river name Rolla (now called the Troelva river) and the last element is lag which means "fishing place". The meaning of the river name is unknown (maybe "the shivering").