Gladsaxehus, Medieval castle ruins in Simrishamn Municipality, Sweden.

Gladsaxehus is a medieval castle ruin in Simrishamn Municipality with stone walls that still stand today. The remains show arrow slits and indicate a fortified structure that once served as both a residence and administrative center.

The site developed in medieval times as a fortified residence for a local noble family. By 1398 it became connected to a Dominican convent endowment, but later evolved into a royal stronghold.

The name Gladsaxehus survives in the 1322 charter form "Glathsyas", written for the church of Saint Jacob. The castle was an independent fief until 1621, when it was merged into Kristianstad County and the king exchanged the castle and most of the estate to Jacob Beck, priest of Gladsax.

The ruin lies just east of Gladsax church and cemetery, reachable on foot from the village. Only the foundations of the stone building remain, so there is little to climb and no marked trail; the ground is grass and uneven. The site is an open monument in Fornminnesregistret, with no facilities or staff on site.

Ide Pedersdatter Falk's 1398 will left her Gladsax manor and a large fortune to found a Dominican nunnery, naming Queen Margaret as executor. After Falk died in 1399, the queen and Bishop Peder of Roskilde obtained papal permission to move the endowment to Gavnø on Zealand instead, keeping Gladsax as a crown counterweight to Åhus and Hammershus.