Lake Sarnen (German: Sarnersee) is a lake in the Swiss canton of Obwalden . The lake is on the Sarner Aa, which flows out of the Lake Lungern , through the Lake Sarnen, and into Lake Lucerne . The municipalities of Sarnen and Sachseln are located on the shores of the lake, and the Brünig railway line follows the eastern shore.

The lake's area is about 7.5 km2 (2.9 sq mi) and its maximum depth is 51 m. It is about 5.7 km (3.5 mi) long and, at its widest, 1.6 km (0.99 mi) wide.

The lake is the subject of a watercolour painting by J. M. W. Turner, The Sarner See, Evening, c.