Mjørkadalur is a vally on the Faroese island Streymoy in the Tórshavnar municipality. It has no postal code. It is located on the mountain 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 for prisoners who serve short sentences. Criminals from the Faroe Islands who serve longer sentences are sent to prisons in Denmark, as there is no prison in the Faroe Islands, only a detention, which has room for 12 persons.