Björksund Castle, Baroque château in Nyköping Municipality, Sweden

Björksund Castle is a Baroque manor house on a ridge by Sibbofjärden, northeast of Nyköping. The main building, completed in 1727 to designs by Joseph Gabriel Destain, has three full storeys on the southwest side and two on the northeast, with a central pediment and a facade clock. Two low wings by Carl Hårleman (1740–1741) flank it, and the facades are painted lion-yellow with white pilasters and light-blue surrounds at the entrances.

Björksund began as part of the Sundboholm estate, whose first known owner was Ingevald Estridsson; in 1522 the estate was divided and Björksund passed to Göran Eriksson Gyllenstierna, husband of Kerstin Grip. The Gyllenstierna family held it until 1776, when it passed partly by purchase to Count Carl Gabriel Mörner. The main building was erected in 1727 to designs by Joseph Gabriel Destain, and the wings were added in 1740–1741 by Carl Hårleman.

The manor's name comes from the two farms, Nälsäter and Flogårdshammar, that were merged to form the säteri of Björksund. The wings once housed the kitchen, pantry, bakery and dairy, while today both wings are dwellings. The estate still arranges several hunts each year.

Visitors should contact Björksund Förvaltnings AB directly to learn about access to the grounds. It is wise to reach out in advance since accessibility may vary depending on the season.

In 1719, as Russian raiders burned the nearby Hälgö estate, Björksund's inspector saved the manor by a ruse: men with horns, old drums and white sheets raised on poles, looking like flags, frightened the enemy into returning to their galleys.