Kastelholm Castle ( Kastelholms slott in Swedish) is a medieval castle in Sund , Åland , Finland . The castle was built on a small island that was surrounded by water and moats filled with several lines of poles. It was significant in the history of Åland and serves today as a tourist attraction.

Kastelholm Castle was first mentioned in 1388 in the contract of Queen Margaret I of Denmark, where a large portion of the inheritance of Bo Jonsson Grip was given to her. The heyday of the castle was in the 15th and 16th centuries . King John III kept his deposed brother Eric XIV in captivity in the castle in the Fall of 1571. The castle was damaged severely when the forces of king Charles IX conquered it in the 1599 civil war. The castle was repaired by 1631, but it burned down in 1745 and was abandoned to ruin in 1770s.

Today, much of the Kastelhom Castle is reconstructed, and it is a major tourist attraction easily accessible by car or bus from Mariehamn .