Saxholmen Castle, Medieval castle ruins on Saxholmen island, Kristinehamn, Sweden

Saxholmen Castle is a ruined medieval stronghold on the island of Saxen in the inlet of Ölmeviken, northeastern Lake Vänern. The site covers about 130 by 30–90 metres and consists of a turf-covered ruin mound, roughly 20 metres across and 3–3.5 metres high, surrounded by five earthen ramparts. The mound has a central pit about 7 metres wide and up to 1.5 metres deep, with a small vault visible at its southern edge. The steep cliffs of the island served as natural defences.

The castle on Saxen is dated to the second half of the 13th century by its finds and construction. Excavations in 1992–1996 found a square stone keep 10 by 10 metres, dwellings, stores and a smithy, with a lower wall and timber palisade around the centre. No written source names its builder; a local lawman, Höld, has been proposed.

Excavations on the island have uncovered items showing how people lived and worked in the castle during medieval times. These discoveries reveal everyday details about how the stronghold functioned as a community.

The island lies in the inlet of Ölmeviken in north-eastern Lake Vänern, in Kristinehamn Municipality. The surrounding rock drops steeply to the west, north and east, and five earthen ramparts, 10 to 25 metres long, block the gaps in the cliff. The ruin mound is about 20 metres across and 3 to 3.5 metres high, with a 7-metre-wide hollow at its centre.

A stone structure at the island's south-eastern tip may be a pier or breakwater; because the island is exposed to hard weather, it may have served to break the waves. Excavations in 1992–1996 found about 2,000 objects at the castle.