Alsnö hus is the ruin of a summer palace at Hovgården, on the island of Adelsö, in Sweden. It is first mentioned in a document from around 1200, as a large house ("mansionem regiam Alsnu"). It was rebuilt by Magnus Ladulås in brick, and the Alsnö stadga was made here in 1279, often said to be the foundation of the Swedish nobility as a separate social class and the start the Swedish feudal system.