The Mainau Castle was a medieval hilltop castle on Mainau Island in Lake Constance, part of the Überlinger See in the district of Konstanz, Baden-Württemberg, Germany. The ministerial family named after the island was first mentioned in 1242. In 1271, the castle came under the ownership of the Teutonic Order, becoming the administrative center for their possessions around Lake Constance, including Mainau Island. From the early 16th century, the lords of Mainau also owned Blumenfeld Castle.
In the 18th century, the castle was demolished, and between 1739 and 1746, the current Baroque-style Mainau Castle was built on the castle ruins by the Teutonic Order's master builder, Johann Caspar Bagnato. The original castle site now only shows traces of the old structure.
