Mjørkadalur is a valley on the Faroese island of Streymoy in the Tórshavnar municipality. It has no postal code. It is located on the mountain of Sornfelli above the fjord of Kalbaksfjørður. The buildings in Mjørkadalur used to be part of the Island Command Faroes, but now a part of the building is used as a detention centre for prisoners who serve short sentences. Criminals from the Faroe Islands who serve longer sentences are sent to prisons in Denmark, as the detention centre only has room for 12 inmates.