Euthydemus (Greek: Εὐθύδημος) is the name of three characters in Socratic literature.
In Book I of the Memorabilia, Xenophon relates Critias' passion for the young Euthydemus and how Socrates mocked him for it: Socrates had observed that Critias loved Euthydemus. Therefore Socrates tried to argue him out of it, saying that it was degrading for a free man and ill became someone "beautiful in body and mind" to importune, moreover for nothing good, his beloved to whom he should be a shining example. Critias, an Athenian sophist and politician, was the leader of the Thirty Tyrants who after the Peloponnesian War ruled for a short while over Athens c. 404 BC.
Another Euthydemus is the eponymous character in one of Plato's dialogues, Euthydemus, written on logic and logical fallacies, or sophisms. The characters Euthydemus and his brother Dionysodorus are sophists questioned by Socrates in a confrontation of the Euthydemian eristic and the Socratic elenchus.
A further Euthydemus is mentioned in Plato's Republic as the son of Cephalus.