The Holy Loch is a body of water in Argyll and Bute, Scotland. Open to the Firth of Clyde at its eastern end, the sea loch is approximately one mile wide and between two and three miles long, varying with the tide. The town of Dunoon on the Cowal peninsula lies on the loch's south western shore and continues round the point to the western shore of the Clyde.

During the Second World War the loch was used as a submarine base, served by the depot ship HMS Forth . An anti-submarine boom was constructed between Dunoon and the Cloch lighthouse to defend waters from German U-boats. The vessels HMS Vandal and HMS Untamed also defended the Holy Loch, along with HMS Graph , a captured German U-boat.

Between 1960 and 1991, the Holy Loch was the site of a U.S. naval base and home to the Polaris nuclear fleet. As a result, it was the location of frequent demonstrations by the Campaign for Nuclear Disarmament and others opposed to the siting of nuclear arms on UK territory.

The name Holy Loch is believed to date from the 6th century when Saint Munn landed there after leaving Ireland.