Ekolsund Castle is a manor house situated at Enköping Municipality in Uppsala County, Sweden.

It is a former royal château, a well-preserved baroque building with 18th-century character.

The current building consists of two wings designed in the second half of the 17th century by Simon de la Vallée and Nicodemus Tessin.

In 1747, it was granted to the future King Gustav III of Sweden, who often used it as his summer residence until he acquired Drottningholm Palace in 1777, especially as Crown Prince.

In 1785, Gustav III sold it to the Seton family, who owned it until 1912.