The Aare is the longest river entirely within Switzerland.
The river rises in the Bernese Alps and runs for roughly 292–295 kilometers before joining the Rhine near Koblenz in the canton of Aargau. Although it flows into the Rhine, the Aare’s entire course remains inside Switzerland.
The Aare passes through or beside major Swiss landmarks, including Lakes Brienz, Thun, and Biel, the city of Bern, and the Aare Gorge. Its important tributaries include the Reuss and Limmat, which join it near Brugg.
A common mix-up is the Rhine: it is longer and borders or crosses several countries, so it cannot be the answer to “entirely within Switzerland.” The Rhône also leaves Switzerland, while the Reuss is a major Aare tributary rather than the country’s longest wholly domestic river.