Sør-Trøndelag is a county in the area Trøndelag in Norway, bordering Nord-Trøndelag, Møre og Romsdal, Oppland and Hedmark. To the west is the Norwegian Sea (Atlantic ocean), and to the east is Sweden. The county is separated into a northern and southern part by Trondheimsfjorden. Slightly over 200 000 of the population lives in Trondheim and its suburbs. The Norwegian dialect of the region is Trøndersk.
The broad and long Trondheimsfjord is at the center of this county, although the coastal areas stretch somewhat further north. The mountain ranges Dovrefjell and Trollheimen are located in the south, while the Fosen peninsula is located north of the fjord. Several of the best salmon rivers in Europe are located in the county, the largest and most famous being Gaula and Orkla. Dovrefjell-Sunndalsfjella National Park, Forollhogna National Park, Skarvan og Roltdalen National Park and Femundsmarka National Park are located, or partly located, in the county.