The Aare (Swiss Standard German: [ˈaːrɛ] ) or Aar (Swiss Standard German: [aːr] ) is the main tributary of the High Rhine (its discharge even exceeds that of the latter at their confluence) 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 kilometres (183 mi), during which distance it descends 1,565 m (5,135 ft), draining an area of 17,779 km2 (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.

The river's name dates to at least the La Tène period, and it is attested as Nantaror "Aare valley" in the Berne zinc tablet.

The name was Latinized as Arula/Arola/Araris.

The Aare rises in the great Aargletschers (Aare Glaciers) of the Bernese Alps , in the Canton of Bern and west of the Grimsel Pass .