Hörningsholm Castle, Castle in Södertälje Municipality, Södermanland County, Sweden

Hörningsholm Castle stands on a cliff above the Borgsund inlet, south of Södertälje. The visible building is a long yellow block of three storeys with evenly spaced windows and a low dark hipped roof, resting on a curved pale stone base with small square openings set on bare rock. Two square single-storey wings flank it, and the medieval ramparts were rebuilt as a terrace.

The structure was built in the late 15th century under Nils Bosson Sture as a defensive stronghold. Russian troops set fire to it in 1719 during military campaigns in the region.

The interior houses an extensive private library and art collection belonging to the Bonde family, representing centuries of Swedish noble heritage.

The castle is a private residence, but visitors can view it from the grounds outside, and the clifftop setting makes for an appealing walk. Access is from the landside approach, though visitors should respect the private nature of the property.

In 1824 a dungeon was found beneath the floor of a basement room, with semi-decomposed human remains inside; it had clearly served as a prison. The silver hoard found in late summer 2025 about 600 m northeast of the castle, near the Näsbacken croft, weighed some 6.25 kg and contained nearly 15,800 Gotland pennies.