The Holy Loch (Scottish Gaelic: An Loch Sianta/Seunta) is a sea loch, a part of the Cowal peninsula coast of the Firth of Clyde, in Argyll and Bute, Scotland.
The "Holy Loch" name is believed to date from the 6th century, when Saint Munn landed there after leaving Ireland. Kilmun Parish Church and Argyll Mausoleum is said to stand where Saint Munn's church was once located.
Robertson's Yard at Sandbank, a village on the loch, was a major wooden boat building company in the late 19th and early 20th centuries.
During World War II, the loch was used as a British Royal Navy submarine base. From 1961 to 1992, it was used as a United States Navy Ballistic missile submarine base. In 1992, the Holy Loch base was deemed unnecessary following the demise of the Soviet Union and subsequently closed.