Reichenau Island is an island in Lake Constance in Southern Germany. It lies almost due west of the city of Konstanz, between the Gnadensee and the Untersee, two parts of Lake Constance. With a total land surface of 4.3 km2 (1.7 sq mi) and a circumference of 11 km (6.8 mi), the island is 4.5 km (2.8 mi) long and 1.5 km (0.93 mi) wide at its greatest extent. The highest point, the Hochwart, stands some 43 m (141 ft) above the lake surface and 438.7 m (1,439 ft) above mean sea level.

Lake Constance refers to three bodies of water on the Rhine at the northern foot of the Alps: Upper Lake Constance (Obersee), Lower Lake Constance (Untersee), and a connecting stretch of the Rhine, called the Lake Rhine (Seerhein). These waterbodies lie within the Lake Constance Basin in the Alpine Foreland through which the Rhine flows.