Hallwyl Castle (; ) is one of the most important moated castles in Switzerland. It is located on two islands in the River Aabach, just north of the northern end of Lake Hallwil in the municipality of Seengen in the canton of Aargau. Since 1925, it has been open to the public, and since 1994 it has been owned by the canton of Aargau and is part of the museum of Aargau.