Forstegg Castle, Medieval castle ruin in Sennwald, Switzerland.

Forstegg Castle is a ruined hilltop castle in Salez, part of Sennwald in the canton of St. Gallen. Its core is a hexagonal tower with 2.1 m thick walls and at least four storeys, built on a roughly 10 m high rock block. The Palas was destroyed by fire in 1894 and removed; a 17th-century armoury (rebuilt in 1790), a barn and remnants of the ring walls and 17th-century schanzen survive.

Forstegg was built around 1200 by the Barons of Sax, probably alongside Hohensax, to secure their holdings in the Rhine valley. The 1248 partition gave it to Ulrich II von Sax, and by the mid-14th century a Hohensax-Forstegg line held it with Büsmig, Frümsen, Haag and Salez. Zurich bought lordship and castle in 1615 and kept a Landvogt seat in the Palas from 1625; the Palas burned in 1894 and was demolished.

The name Forstegg comes from the Sax-Forstegg branch of the Barons of Sax, who split from Sax-Hohensax in the mid-14th century under Ulrich IV Stephan (1329–1381). Their lordship covered Büsmig, Frümsen, Haag and Salez, and in 1396 Ulrich Eberhard IV also bought the village and court of Sennwald. From 1615 Zurich administered the castle as the Landvogtei Sax-Forstegg, and in 1625 a Landvogt's residence was set up in the Palas.

The ruin is on private land in the Salez section of Sennwald, on a hill about 460 m above sea level at the edge of the Schlosswald forest. The tower stands on a roughly 10 m high rock block, and the surviving 17th-century armoury and a barn are on the grounds. There are no visitor facilities or marked trails recorded; ask locally before entering.

The castle keep stands on a roughly 10-metre-high rock block, and its walls are 2.1 metres thick. The tower had at least four storeys from the outset and a hexagonal ground plan. Around the site, remains of the 17th-century ring walls and schanzen survive.