Fosen is a traditional district in Trøndelag, consisting of the municipalities Osen, Roan, Åfjord, Ørland, Indre Fosen, Orkland, Heim, Hitra and Frøya. The district is dominated by forested valleys, lakes, coastal cliffs but also shallow areas, and in the interior mountains reaching up to 675 m elevation. The western coast has many skerries and some islands, such as Stokkøya in Åfjord. There are some good salmon rivers, and sea eagles and other sea birds are very common along the coast, notably on the shallow area near Ørland (Grandefjæra). The west coast has mild winters, and some locations receive on average more than 2,000 mm of precipitation per year. Part of the Scandinavian coastal conifer forests are located in the valleys of the peninsula, and smaller areas are classified as temperate rainforest with 67 nature reserves. The largest nature reserve is Øyenskavelen, with many nature types including undisturbed forest, some of it classified as rainforest.

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.