Frassilongo (Mocheno: Garait) is a comune (municipality) in Trentino in the northern Italian region Trentino-Alto Adige/Südtirol, located about 14 km east of Trento. As of 31 December 2004, it had a population of 354 and an area of 16.7 km2. Frassilongo borders the following municipalities: Sant'Orsola Terme, Fierozzo, Roncegno, Vignola-Falesina, Pergine Valsugana, Novaledo and Levico Terme. In the census of 2001, 340 inhabitants out of 357 (95.2%) declared themselves members of the Mócheno linguistic group.

Demographic evolution