The Theater St. Gallen is a performing arts center for opera, musical, ballet, and theatre in St. Gallen, Switzerland, and is considered to be the oldest professional theatre in Switzerland. It is a Swiss heritage site of national significance.

Although the monks Tutilo and Notker already inspired briefly lay theatre in St. Gallen during the Middle Ages, the theatre did not flourish for a long time because of moral concerns. In the early 19th century interest in theatre was reawakened. After the first performance on October 14, 1801, in modest surroundings, a "Theater Stock Company" was founded in 1805 to support a local professional theatre group. When they needed a larger place for their performances, the architect Johann Christoph Kunkler built a theater into which the group moved in 1857. This "Kunkler-Bau" remained the home of the formerly called Stadttheater, St. Gallen, for 111 years until 1968. The building was then demolished.