A three-armed reservoir in the centre of the Cowal Peninsula of Argyll and Bute, Loch Tarsan lies next to the B838 road 7½ miles (12 km) northwest of Dunoon. The reservoir was created in 1951 by flooding the middle section of Glen Tarsan, also extending into Glen Lean, retained by two substantial buttress dam walls, one within each valley.
It extends to 110 ha (273 acre) and provides water as part of the Cowal Hydro-Electric Power Scheme. The loch is stocked with brown trout and fishing is available from the bank or using boats.