Kapıdağ Peninsula (Turkish: Kapıdağ Yarımadası) is a tied island in northwestern Anatolia extending into the Sea of Marmara in Balıkesir Province, Turkey. The peninsula forms the Gulf of Bandırma on its east and the Gulf of Erdek on its west.
Kapıdağ was the classical island of ancient Greek Arctonnesus, but was joined to the mainland by a narrow isthmus in historic times either by an earthquake or (according to legend) by Alexander the Great. It was also known as the Peninsula of Cyzicus after its chief town.