Rollag is a municipality in Buskerud county, Norway. It is part of the traditional region of Numedal. 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 in Telemark.

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 municipality of Rollag was established on 1 January 1838 (see formannskapsdistrikt). Nore og Uvdal was separated from Rollag in 1858. The coat-of-arms is from modern times. They were granted in 1993. The arms show two gold-colored lines on a red background. The lines represent an old warning cairn made of timber.

The municipality lies in the heart of the Numedal traditional region and valley, the westernmost valley in southeastern Norway. The river Numedalslågen flows through Rollag and into the Ytre Oslofjord at Larvik. In the northwest of the Rollag lie the Vegglifjell mountains, which are an entryway into the Hardangervidda. The municipality's highest point is Storegrønut at 1,289 metres (4,229 ft) located in the Vegglifjell mountains.

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