Umbagog Lake is a Lake in Coos, New Hampshire.

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Umbagog Lake is a wilderness lake located in Coös County, New Hampshire and Oxford County, Maine. It is one of the most pristine lakes in the state of New Hampshire and lies near Errol, New Hampshire and the Thirteen Mile Woods Scenic Area along the border with Maine.

The lake is part of a National Wildlife Refuge and a New Hampshire state park. Along its southernmost shore, there is a public campground and a public boat launch ramp which may be accessed from New Hampshire Route 26. An interesting feature along its northwest shore is an expansive natural floating island composed of generations of decomposing marshland vegetation.

The lake runs almost 11 miles (18 km) north to south. Its surface area is 7,850 acres (31.8 km2), making it the largest lake along the Maine/New Hampshire border. Its average depth is less than 15 feet (4.6 m), and its maximum depth is 48 feet (15 m). A deep section at the north end (aptly named "Deep Hole") may exceed 75 feet (23 m). The lake's area and depth were markedly increased with the construction of a dam at Errol in the 1800s.

Umbagog Lake is fed by the Magalloway River, the Rapid River, and the Dead Cambridge River. It is the source of the Androscoggin River.