Plansee is a lake in Reutte District , Tyrol, Austria, located at 47°28′10″N 10°48′20″E. Its surface is approximately 2.87 km² and its maximum depth is 78 metres.

It lies on Austrian Federal Highway B 179, which crosses the Ammersattel into Germany.

Plansee and Heiterwanger See are probably the remains of an Ice Age meltwater reservoir that once covered the entire Zwischentoren (de:Zwischentoren) area up to the Ehrwald Basin (de:Ehrwalder Becken).

The lake is first mentioned in a border description of the diocese of Freising from the years 1073/78 as Planse, other early names include; Please , Plense and Plase.

The Tyrolean princes and the Steingaden monastery held the fishing rights on the Plansee, as the Stams monastery in the 15th century through gift or acquisition.