Isagoras (Ἰσαγόρας), son of Tisander, was an Athenian aristocrat in the late 6th century BC.
He had remained in Athens during the tyranny of Hippias, but after Hippias was overthrown he became involved in a struggle for power with Cleisthenes, a fellow aristocrat. In 508 BC he was elected archon eponymous, but Cleisthenes opposed him, with support from the majority of the population. Isagoras requested support from the Spartan king Cleomenes I, an old friend who had earlier been given hospitality by Isagoras; according to Herodotus. Cleomenes had also had an affair with Isagoras' wife.
Isagoras, with Cleomenes' help, expelled Cleisthenes and other members of the Alcmaeonidae family on pretext of the Alcmaeonidaean stain (see Megacles). Cleisthenes' supporters resisted Isagoras and Cleomenes and trapped them on the Acropolis for two days. On the third day they made a truce, allowed Cleomenes and Isagoras to escape, and executed 300 of Isagoras' supporters. Cleisthenes then returned to the city and became archon.