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.

[4] 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 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.