Lake Tanzawa, Mountain reservoir in Kanagawa Prefecture, Japan.

Lake Tanzawa is a reservoir in the mountains of Kanagawa Prefecture, about 70 kilometers west of Tokyo. The shoreline runs between forested slopes covered mainly in cedar and pine, with several coves holding small gravel beaches in deeper sections.

The dam was completed in 1978 to secure drinking water for the Shonan region and Yokohama. In the following years, walking paths and rest areas were built around the water to open the area for visitors seeking recreation.

The drowned village of Yozuku gave its name to the Hyakumanben Nenbutsu, a 600-year-old rite at Nōan-ji, moved to Mukaihara in 1978 and held each mid-February. A great rosary of 302 beads, about 9 metres long, is strung on a pulley and turned while elders count the rotations and chanters answer in three registers. The Tanzawa Lake Memorial Hall keeps the Ōzaki Jōmon pottery and the tools of the former Miho hamlets.

Canoes and SUPs require an application at the Lake Tanzawa Memorial Hall, with boarding at the Yajitsu boat landing, the lake's only one; SUP use runs April to November. In summer and early autumn, when the water level drops, the landing may move beneath Eisai Bridge. Rental bicycles at the memorial hall are available to children from fourth grade up, with routes to the Chiyonosawa overlook and the dam crest.

The reservoir holds 64.9 million cubic metres and supplies about 30% of Kanagawa Prefecture's tap water. The Ozaki archaeological site, a Jōmon settlement, was excavated in 1976 before being submerged; its stone axes are displayed at the Lake Tanzawa Memorial Hall.