Megacles, the grandson of another Megacles and member of the Alcmaeonidae family, was an opponent of Pisistratus in the 6th century BC. He drove out Pisistratus during the latter's first reign as tyrant in 560 BC, but the two then made an alliance with each other, and Pisistratus married Megacles' daughter. They also tricked the Athenians into believing Athena herself had arrived to proclaim Pisistratus tyrant, by dressing up a woman named Phye as the goddess. However, Megacles turned against Pisistratus when Pisistratus refused to have children with Megacles' daughter, which brought an end to the second tyranny.
Megacles competed with Hippocleides to marry Agarista, the daughter of Cleisthenes of Sicyon. His son with Agarista, also named Megacles, was the grandfather of yet another Megacles, who himself was the uncle of Pericles.