Södertuna Castle is a hotel and former castle in Gnesta Municipality, Sweden. It dates to the 14th century and is now a luxury hotel and conference center. The main building is of medieval origin, but was updated during the 17th century and the late 18th century. The last major renovation was in 1892. The castle is situated on an island in Frösjön not far from Gnesta. It has belonged to members of several families: Sparre, Lovisin, Palmenfelt, Ehrensvard, Wachtmeister, Adelborg, and von Eckermann.