Semerwater (/ˈsɛmər-/) is the second largest natural lake in North Yorkshire, England, after Malham Tarn. It is half a mile (800 m) long, covers 100 acres (0.40 km²) and lies in Raydale, opposite the River Bain within Yorkshire Dales National Park. A private pay and display parking area is at the foot of the lake.

Semerwater attracts canoers, windsurfers, yachtsmen and fishermen. There are three small settlements nearby:

Semerwater was the subject of a number of sketches and paintings by the artist J. M. W. Turner.

Semerwater is a pleonastic place name. The name, first recorded in 1153, derives from the Old English elements sæ 'lake', mere 'lake' and water. The form "Lake Semerwater" introduces a fourth element with the same meaning.

The lake is a Site of Special Scientific Interest, first notified in 1975.

According to an old legend, Semerwater was once occupied by a prosperous city. One night an old man (or in some versions, an angel in disguise) came down to the city, in search of food and drink. He went from door to door, and at each house he was turned away. Finally, he came to the hovel of a poor couple just outside the town; the couple took him in and treated him with great kindness.

When the stranger was about to leave, he turned to face the town and uttered the curse:

Semerwater rise, and Semerwater sink,
And swallow the town all save this house,
Where they gave me food and drink.