Megalonisi (, "big island") is an uninhabited islet off the southern coast of the Greek island of Crete in the Libyan Sea that hosts a lighthouse. The islet is in a bay between the capes of Lithino and Kefalas, at Kommos, and close to Gortyn which was the ancient capital of Crete. It is administered within Heraklion regional unit.
There is a group of four islets in the bay including Papadoplaka (to the west), Megalonisi (with the lighthouse), Mikronisi (also known as Agios Pavlos), and Trafos.