Cleisthenes (also Clisthenes or Kleisthenes) was the tyrant of Sicyon, who aided in the war against Cirra that destroyed that city in 595 BC. He organized a competition with his daughter Agarista as a prize; the two main competitors for her were the Alcmaeonid Megacles, and Hippocleides. Because Hippocleides made a fool of himself in front of Cleisthenes, Megacles was chosen to marry Agarista.

There was also another Cleisthenes, a noble Athenian who reformed the constitution of ancient Athens and set it on a democratic footing in 508 BC. He was a relative of the above Cleisthenes, through Megacles and Agarista.

With help from the Alcmaeonidae (Cleisthenes' genos) and the Spartans, he was responsible for overthrowing Hippias, the son of the tyrant Pisistratus. As soon as he came to power he revised the citizens' lists to remove all the "new" citizens, those who had immigrated to Athens during the time of Solon. He also decreed that all of the other citizens of Athens would be his hetairoi, his "companions," meaning the entire city could now participate in government, rather than a small group of aristocrats. This was opposed by the archon Isagoras, who appealed to the Spartan king Cleomenes I. Isagoras and Cleomenes were able to exile Cleisthenes and all of the other Alcmaeonidae, but the newly enfranchised citizens rebelled against this, exiling Isagoras and allowing Cleisthenes to return.

After his victory Cleisthenes began to reform the government of Athens. He eliminated the four traditional tribes, which were based on family relations and had led to the tyranny in the first place, and organized citizens into ten tribes according to their area of residence (their deme). There were probably about 130 demes, organized into thirty groups called trittyes, with ten trittyes divided among three regions in each deme (a city region, Pedia; a coastal region, Peralia; and an inland region, Mesogeia). He also established legislative bodies run by individuals chosen by lot, rather than by kinship or heredity. He reorganized the Boule, created with 400 members under Solon, so that it had 500 members, 50 from each tribe. The court system was reorganized so there were 5000 jurors selected each day, 500 from each tribe.

Cleisthenes also seems to have introduced ostracism (first used in 487 BC), whereby the citizens voted to exile a citizen who was felt to have too much power in the city. Under this system, the exiled man's property was maintained, but he was not physically in the city where he could possibly create a new tyranny.

Cleisthenes called these reforms isonomia ("equality under the law"), rather than democratia.