Frischenberg is a ruined castle in Sennwald, canton of St. Gallen, Switzerland.

It is located on a rocky ridge at 585 m (1,919 ft) elevation, overlooking the Alpine Rhine Valley.

It was bought by Ulrich von Hohensax in 1440, but in 1446, the castle was again attacked by Appenzell, in the context of the Old Zürich War, and was destroyed.

After the St. Gallerkrieg of 1489/90 it passed to the Swiss Confederacy, who gave it to Ulrich IX of Hohensax.

The castle was abandoned by Ulrich Philipp von Hohensax, who built his new residence in Sax (now part of Sennwald municipality).