Kapıdağ Peninsula (Turkish: Kapıdağ Yarımadası) (Greek: Χερσόνησος της Κυζίκου) is a tied island in northwestern Anatolia extending into the Sea of Marmara in Balıkesir Province, Turkey.

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.

After its conquest by the Ottoman Empire, it was part of the kaza of Erdek in the province of Bursa.

Erdek hosts a naval base of the Turkish Navy.

This geographical article about a location in Balıkesir Province, Turkey is a stub.