Lake Motosu (本栖湖, Motosu-ko) is the westernmost of the Fuji Five Lakes and located on the border of the towns of Fujikawaguchiko and Minobu in southern Yamanashi Prefecture near Mount Fuji, Japan.
Lake Motosu is the third-largest of the Fuji Five Lakes in terms of surface area, and is the deepest, with a maximum water depth of 121.6 metres (399 feet), making it the ninth-deepest lake in Japan. Its surface elevation of 900 metres (3,000 feet) is the same as for Lake Shōji and Lake Sai, confirming that these three lakes were originally a single lake, which was divided by an enormous lava flow from Mount Fuji. The remnants of the lava flow are now under the Aokigahara Jukai Forest, and there is evidence to indicate that these three lakes remain connected by underground waterways. The temperature of the water never drops below 4 °C (39 °F), making it the only one of the Fuji Five Lakes that does not freeze in winter.
The lake is within the borders of the Fuji-Hakone-Izu National Park.As with the other Fuji Five Lakes, the area is a popular resort, with many lakeside hotels, windsurfing facilities, camp sites, and excursion boats. Rainbow trout and brown trout were introduced to the lake in the Meiji period, and sports fishing is also popular. However, in recent years, reduced water transparency due to pollution from these activities has been a growing issue.
The lake and its view of Mount Fuji were featured on Series D of the Japanese 5000-yen note and Series E of the Japanese 1000-yen note.