Megacles, the grandson of the late 7th century Megacles, son of Alcmaeon and member of the Alcmaeonidae family, is an opponent of Pisistratus in the 6th century BCE.
He drives out Pisistratus during the latter's first reign as tyrant in 560 BCE, but the two then make an alliance with each other, and Pisistratus marries Megacles' daughter.
Herodotus claims that 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.
This event is subject to debate as to whether Herodotus has interpreted this episode correctly.
However, Megacles turns against Pisistratus when Pisistratus refuses to have children with Megacles' daughter, which brings an end to the second tyranny.
This Megacles later competes circa 560 BCE or later with Hippocleides, a former archon of Athens, to marry Agarista, the daughter of Cleisthenes of Sicyon.
They have two sons; the elder Hippocrates is father of another Megacles (ostracized 486 BC) and a daughter Agariste is mother of Pericles and Ariphron (himself the father of Hippocrates of Athens who dies in 424 BC).
The younger son Cleisthenes is allegedly father of Deinomache (or Dinomache), mother of Alcibiades (d. 404 BCE).
Thus, Megacles the elder is great-grandfather of Pericles and Alcibiades.