Cleisthenes (active 6th century BC) was an Athenian political leader and constitutional reformer. The first avowed democratic leader, he introduced important changes into the Athenian constitution.

Son of Megacles, leader of the powerful Alcmeonid clan in Athens, and of Agariste, daughter of Cleisthenes, the tyrant of Sicyon, Cleisthenes was destined for a public career. Accommodating himself to the regime of the tyrants, he was chief magistrate of Athens in 525 B.C., but he and other Alcmeonids were in exile when the tyranny fell in 510.

Cleisthenes ran for leadership of Athens at the head of a noble faction favoring oligarchy; he was defeated by Isagoras, a friend of the Spartan king Cleomenes. Cleisthenes then turned democrat, threatening the position of Isagoras, who asked Cleomenes for help. The Spartan king arrived with troops and tried to disband the Council of 300 and install Isagoras as head of a new council, but...