Cleisthenes (Greek: Κλεισθένης, also Clisthenes or Kleisthenes) was a noble Athenian of the Alcmaeonid family. He is credited with reforming the constitution of ancient Athens and setting it on a democratic footing in 508/7 BC. For these accomplishments, historians refer to him as "the father of Athenian democracy." He was the maternal grandson of the tyrant Cleisthenes of Sicyon, as the younger son of the latter's daughter Agariste and her husband Megacles. Also, he was credited for increasing power of assembly and he also broke up power of nobility for Athens.

Historians estimate that Cleisthenes was born in ~570 BC. Cleisthenes was the uncle of Pericles' mother Agariste and of Alcibiades' maternal grandfather Megacles.

With help from the Alcmaeonidae (Cleisthenes' genos, "clan"), he was responsible for overthrowing Hippias, the tyrant son of Pisistratus. After the collapse of Hippias' tyranny, Isagoras and Cleisthenes were rivals for power, but Isagoras won the upper hand by appealing to the Spartan king Cleomenes I to help him expel Cleisthenes. He did so on the pretext of the Alcmaeonid curse. Consequently, Cleisthenes left Athens as an exile, and Isagoras was unrivaled in power within the city. Isagoras set about uprooting hundreds of people from their homes on the pretext that they too were cursed, and attempted to dissolve the council (βουλή, boulê). However, the council resisted, and the Athenian people declared their support of it. Hence Isagoras and his supporters were forced to flee to the Acropolis, remaining besieged there for two days. On the third, they fled and were banished. Cleisthenes was subsequently recalled, along with hundreds of exiles, and he assumed leadership of Athens.