Karlstor, Medieval gate in St. Gallen, Switzerland.

Karlstor is the only surviving gate of St. Gallen's medieval city wall, built in 1569 and 1570 by master builder Carolus Briegel. Above the pointed-arch passage sit three rows of narrow windows from its use as a prison, and a large stone relief covers the upper two storeys of the east facade, flanked by two stone lion heads.

Built in April 1569 by master builder Carolus Briegel and finished around August 1570, the gate followed the 1566 Treaty of Wil, which let Abbot Otmar Kunz break his own entrance through the shared wall after Abbot Ulrich Rösch's earlier request had failed. It was first called Neues Tor, Gotteshaustor or Abtstor, and took its present name from Cardinal Karl Borromäus, who rode through it on 26–27 August 1570 as the first high cleric to enter the city by this gate.

The gate's name recalls Cardinal Karl Borromäus of Milan, who rode through it on 26 and 27 August 1570 as the first high cleric to enter the city by this new gate. Before that it was called Neues Tor, Gotteshaustor or Abtstor, because it served the abbot and his retinue. The chambers above the gate have served as a prison since at least 1812 and today hold the remand prison.

The gate is located in the center of St. Gallen and is easy to reach by public transport. Pedestrians can walk directly to it and explore it from several sides.

This gate is the only surviving remnant of the complete fortification system that once surrounded St. Gallen. This fact makes it a valuable witness to how the city was once defended.