St. Margrethen (Saint Margrethen/Sankt Margrethen) is a municipality in the Wahlkreis (constituency) of Rheintal in the canton of St. Gallen in Switzerland.

It is close to Austria and Germany and acts as a border town with Austria.

Currently only a normal road links St. Margrethen with Austria. As a result, the town is at times clogged up with traffic before the customs posts. A motorway link has long been projected, to link the pathways to Zurich and Munich. It is also where the A1 motorway becomes the A13 motorway.

The Old Church of St. Margaretha is listed as a Swiss heritage site of national significance. Additionally, the Schlosslandschaft Rorschach / Alter Rhein, a number of castles in a region along the Rhine river, is designated as part of the Inventory of Swiss Heritage Sites. The Schlosslandschaft is shared between Berg, Goldach, Mörschwil, Rheineck, Rorschacherberg, St. Margrethen, Steinach, Thal and Tübach.