Mustla is a small borough (alevik) in Viljandi Parish, Viljandi County, Estonia. As of the 2011 census, the settlement's population was 818. 1938–1979 Mustla was a town with town privileges. In June 2018, a plaque commemorating Alfons Rebane was unveiled on the wall of a private building in Mustla where he had lived. The Russian Ministry of Foreign Affairs protested the unveiling.