Pleistoanax (; reigned 458–409 BC) was an Agiad King of Sparta. He was the son of regent Pausanias, who was disgraced for conspiring with Xerxes. Pleistoanax was most anxious for peace during the so-called First Peloponnesian War. He was exiled sometime between 446 BC and 444 BC, charged by the Spartans with taking a bribe, probably from Pericles (noted as "10 talents necessary expenses" in Athens' funds), to withdraw from the plain of Eleusis in Attica after leading the Peloponnesian forces there following the revolts of Euboea and Megara from the Athenian empire. Accepting such a bribe would have essentially amounted to treason, but some scholars (e.g. Walker, Meyer, Beloch, Busolt) doubt this, or at least agree that it is not enough information to explain the happenings. Also some believe that a more probable reason for the withdrawal of Pleistoanax and his advisor Cleandrides could be that Pericles offered good terms for a peace (e.g. later there was a treaty between Sparta and Athens).
In 428 BC, Pleistoanax was recalled and restored in obedience to the Delphic oracle's advice, though some suspected he had tampered with the Pythia. His enemies still blamed him for Spartan disasters, so Pleistoanax advocated peace to bring an end to the disasters.
He was succeeded by his son Pausanias.