The Aare (German pronunciation:[ˈaːrə]) or Aar is a tributary of the High Rhine and the longest river that both rises and ends entirely within Switzerland.

Its total length from its source to its junction with the Rhine comprises about 295 km (183 miles), during which distance it descends 1,565 m (5,135 ft), draining an area of 17,779 km (6,865 sq mi), almost entirely within Switzerland, and accounting for close to half the area of the country, including all of Central Switzerland. There are more than 40 hydroelectric plants along the course of the Aare River.