Utö hus, Medieval castle on Arnö Island in Lake Mälaren, Sweden.

Utö hus is a rectangular stone structure on Arnö Island in Lake Mälaren with characteristic stepped gables. The building contains two main floors, two attic levels, and a cellar with an elevated entrance.

Utö hus is first mentioned in written sources in the early 15th century, when it belonged to the Schack family. In the 1630s the estate was merged with neighbouring Grönsö when Johan Skytte married Maria Näf. Repaired around 1740, it was occasionally inhabited until the 1840s, when it became a grain store. In 1937 Alice von Ehrenheim donated it to the Royal Swedish Academy of Letters, History and Antiquities.

The building shows how Swedish nobles lived during medieval times and which rooms they used daily. The seven ground-floor rooms still preserve their arrangement from the 17th century.

Since 2015 the property has been managed by the National Property Board (Statens fastentingsverk), which took over from the National Heritage Board; it has been a state building monument since 1966. The house stands on Arnö in central Lake Mälaren, so reaching it means crossing water; it can be combined with Grönsö slott on the opposite side of the Grönsösundet strait.

The building sits on one side of the Grönsösundet strait and controlled shipping routes on Lake Mälaren in tandem with Grönsöö Castle across the water. This paired position allowed the owners to monitor all passing traffic.