The Birs (French: Birse) is a 73-kilometre (45 mi) long river in Switzerland that flows through the Jura region and ends as a tributary to the Rhine between Basel and Birsfelden.

The Birs has its source in a spring near the Col de Pierre Pertuis at 762 metres (2,500 ft) above sea level a little southwest of Tavannes in the Jura bernois.

The Birs runs through wider valleys (Vallée de Tavannes) and narrow gorges.

Between Soyhières and Liesberg, it leaves the French-speaking part of Switzerland, enters the canton of Basel-Landschaft and receives the Lützel from the left.

Earlier, the Birs was polluted and dammed, but it has largely been restored to its original state.