Dounreay (Ordnance Survey ) is the name of a now ruinous castle on the north coast of Caithness, in the Highland area of Scotland. The castle, however, is within grounds used by the United Kingdom Atomic Energy Authority (Dounreay Nuclear Power Development Establishment) and the Ministry of Defence (Vulcan Naval Reactor Test Establishment). It is the site of five nuclear reactors, three owned and operated by the UKAEA and two by the Ministry of Defence.

The nuclear power establishment was built on the site of a World War II airfield, called HMS Tern (II). The airfield was transferred to the Admiralty by RAF Coastal Command in 1944, as a satellite of HMS Tern at Twatt in Orkney. It never saw any action during the war and was placed into care and maintenance in 1949.

Dounreay is near the A836 road, about 9 miles (14 km) west of the town of Thurso, which grew rapidly when the research establishment was developed during the mid 20th century. The establishment has remained a major element in the economy of Thurso and Caithness since that period.