Kastelruth (Austrian German: [ˌkastl̩ˈruːt]; Italian: Castelrotto [kaˌstɛlˈrotto]; Ladin: Ciastel) is a comune (municipality) and a village in South Tyrol in northern Italy, about 20 kilometres (12 mi) northeast of the city of Bolzano.

[3] As of 30 November 2010, it had a population of 6456 and an area of 117.8 square kilometres (45.5 sq mi).

[4] Kastelruth borders the following municipalities: Barbian, Campitello di Fassa, Völs am Schlern, Lajen, Urtijëi, Waidbruck, Ritten, Santa Cristina Gherdëina and Tiers.

The municipality contains the frazioni (subdivisions, mainly villages and hamlets) Seiser Alm (Alpe di Siusi), Pufels (lad.

According to the 2024 census, 78.47% of the population speak German, 15.81% Ladin and 5.72% Italian as first language.