Rüthi is a municipality in the canton of St. Gallen in Switzerland.
Rüthi is first mentioned in 820 as Reuti. Until 1994 it was known as Rüthi (Rheintal).
Rüthi has an area, as of 2006, of 9.3 km2 (3.6 sq mi). Of this area, 47.9% is used for agricultural purposes, while 33.8% is forested. Of the rest of the land, 13.7% is settled (buildings or roads) and the remainder (4.6%) is non-productive (rivers or lakes).
The municipality was located in the Oberrheintal district until the creation of the Rheintal Wahlkreis. It consists of the villages of Rüthi, Büchel and the Hirschensprung as well as the southern part of the hamlet of Rehag. Other villages were part of the municipality. From 1798 to 1831 the village of Kobelwald (later part of Oberriet) and from 1803 to 31 the village of Lienz (afterward part of Sennwald, then in 1883 part of Altstätten) were part of Rüthi.
The blazon of the municipal coat of arms is Azure a Deer salient Or between two Cliffs Argent issuant from flanks and in chief as many Mullets of the same.