Ekolsund Castle, Manor house in Enköping Municipality, Sweden.

Ekolsund Castle is a former royal château in Enköping Municipality, about ten kilometres east of Enköping near Ekolsundsviken. The preserved baroque complex was built in stages from the mid-17th to the mid-18th century, with two wings by Simon de la Vallée and Nicodemus Tessin the Elder. The main block has a steep dark hipped roof, yellow upper walls and a dark red ground floor, with a lower wing to the left.

The estate began as a medieval property in the 14th century and entered royal ownership in 1542. It gained importance when Princess Sophia of Sweden made it her residence from 1578 to 1611, establishing it as a favored home for members of the royal family.

The name Ekolsund is first recorded in two letters written at the estate in April 1307. Gustav III created the Ekolsundsdräkt on Christmas Eve 1772, a dress worn by selected guests at Ekolsund and other royal estates. In August 1776 he held a tournament there; some armour from it is kept in Livrustkammaren.

The estate lies by Enköpingsvägen, about ten kilometres east of Enköping near Ekolsundsviken. The castle is privately owned and not open to the public; the 1770s Ekolsunds värdshus next to the building is itself a listed building. The south wing is used for weddings, company events and film shoots.

On Christmas Eve 1772 Gustav III created the Ekolsund costume, a dress reserved for specially chosen guests at Ekolsund and the other royal estates. In August 1776 he staged a tournament there; some of its armour is kept in the Livrustkammare in Stockholm.