Södertuna Castle, Manor estate on an island in Gnesta Municipality, Sweden.

Södertuna Castle stands on the islet of Fruktön in Lake Frösjön, about 2.5 km north of Gnesta. Its main building rests on medieval cellar vaults, probably from Jon Karlsson Färla's fortified manor of the 1400s. Erik Lovisin rebuilt it in the late 1600s, and Isak Gustaf Clason gave the corps de logi its Neoclassical look in 1892, adding a sandstone portal with festoons and a columned veranda.

The castle was founded in medieval times and underwent major rebuilding during the 17th and 18th centuries. In 1985 it was separated from its parkland, and in 1986 it opened as a hotel following conversion work.

The estate's name was Vad or Wad until the 18th century, when it became Södertuna. The medieval cellar vaults are said to resemble monk cells, and one walled-up door is claimed to lead to an underground passage to the lake shore. The yellow stable on the island now hosts parties and events.

The hotel offers rooms with modern comfort such as LCD televisions and wireless internet connection, each furnished differently. Visitors can enjoy activities like canoeing in summer or ice skating on the frozen lake during winter months.

The medieval vaulted cellars under the main building are not monastic, as local tradition claims, but probably the remains of Jon Karlsson Färla's fortified manor from the 1400s. One cellar has a walled-up door said to lead to an underground passage opening at the lakeshore.