Aristodemus was one of the Heracleidae, the descendants of Heracles. He was the son of Aristomachus and the brother of Cresphontes and Temenus, and a great-great-grandson of Heracles himself.

By his wife Argia, daughter of King Autesion of Thebes, he was the father of the twin kings Eurysthenes and Procles, who became the ancestors of the two royal houses of Sparta.

Aristodemus helped lead the fifth and final attack on Mycenae in the Peloponnese, part of the Heracleidae's long effort to reclaim the land they regarded as their inheritance.

Aristodemus and his brothers complained to the oracle that its instructions had proved fatal to those who had followed them: the oracle had told Hyllus to attack through the narrow passage when the third fruit was ripe. They received the answer that by the "third fruit" the "third generation" was meant, and that the "narrow passage" was not the Isthmus of Corinth but the straits of Rhium. They accordingly built a fleet at Naupactus to make the crossing.

Before the fleet at Naupactus set sail, disaster struck: Aristodemus was killed by lightning (or, in another version, shot by Apollo), and the fleet was destroyed. The cause given was that one of the Heraclidae had slain an Acarnanian soothsayer.

His brothers Temenus and Cresphontes were later able to complete the conquest of the Peloponnese without him.

Although Aristodemus himself did not live to rule, his line endured. His twin sons by Argia, Eurysthenes and Procles, inherited his place among the conquering Heraclidae.

From these twins descended the two parallel royal houses of Sparta, an arrangement by which two kings ruled the city simultaneously — a defining feature of the Spartan constitution traced back to Aristodemus's sons.