Sjöö Castle, Medieval castle in Örsundsbro, Sweden.

Sjöö Castle is a castle in Holm Parish, Enköping Municipality, Uppland. It was built from 1669 to 1679 to designs by Nicodemus Tessin the Elder, modified by Mathias Spieler, for Johan Gabriel Stenbock. Its cross-shaped main-floor hall was later filled in to a rectangular form in the 19th century, and its park was laid out in the 1670s by French specialist Pierre Durant.

The estate is recorded from the Middle Ages; in 1409 Peter Laurensson took it over, and in 1541 Gustav Vasa bought it from bailiff Jacob Vestgöte Sjöö. Erik XIV later transferred it to Gustaf Johansson (Three Roses), and it passed by inheritance to Johan Gabriel Stenbock, who had Nicodemus Tessin the Elder design the castle with Mathias Spieler modifying the plans; construction ran from 1669 to 1679. Stenbock died unmarried, and the estate passed to Hedvig Catharina Lilje and then to the Banér family in 1813.

The exterior of Sjöö Castle shows how Swedish noble architecture borrowed ideas from French baroque design, especially in the symmetry of the facade and the rhythm of its windows. Walking along the public paths around the estate gives a clear sense of how landed families in this part of Sweden used grand buildings to signal their status.

The castle is privately owned and cannot be visited inside, but it can be seen clearly from the public paths that run around the estate. Visiting on a clear day gives the best view of the facade and the park from a respectful distance.

The cross-shaped main hall inside was not part of the original design but was added in the 19th century, drawing on Italian villa architecture. This later addition means the building reflects two distinct periods of construction, not just one.