Sør-Trøndelag was a county comprising the southern portion of the present-day Trøndelag county in Norway. It bordered the old Nord-Trøndelag county as well as the counties of Møre og Romsdal, Oppland, and Hedmark. To the west is the Norwegian Sea, and to the east is Jämtland in Sweden. The county was separated into a northern and southern part by the Trondheimsfjord. Slightly over 200,000 of the county's population lives in the city of Trondheim and its suburbs. The Norwegian dialect of the region is Trøndersk.

Trøndelag (Urban East Norwegian: [ˈtrœ̂ndəˌlɑːɡ]; or Trööndelage  is a county and coextensive with the Trøndelag region in the central part of Norway. It was created in 1687, then named Trondhjem County ; in 1804 the county was split into Nord-Trøndelag and Sør-Trøndelag by the King of Denmark-Norway, and the counties were reunited in 2018 after a vote of the two counties in 2016.