Dunoon Castle is a ruined castle in Dunoon, Argyll and Bute, Scotland. The castle sat upon a cone-shaped hill about 80 feet (24 m) high. Very little remains of the castle's structure today. Castle House, built in 1822, stands a few yards north of the castle ruins.

Dunoon is the main town on the Cowal peninsula in the south of Argyll and Bute, Scotland. It is located on the western shore of the upper Firth of Clyde, to the south of the Holy Loch and to the north of Innellan. As well as forming part of the council area of Argyll and Bute, Dunoon also has its own community council. Dunoon was a burgh until 1976.