Loch Ascog is a small reservoir on the east coast of the island of Bute, in the council area of Argyll and Bute, Scotland. The loch supplies water to the town of Rothesay and the fishing rights are held by the Isle of Bute Angling Association. Loch Ascog is 44.7 hectares in extent. To the west is the much larger Loch Fad.