Dionysodorus (Greek: Διονυσόδωρος, Dionysódōros, c. 430 – late 5th century or early 4th century BC) was an ancient Greek sophistic philosopher and teacher of martial arts, generalship, and oration. Closely associated with his brother and fellow sophist Euthydemus, he is depicted in the writing of Plato and Xenophon.
Plato's Euthydemus features Dionysodorus and Euthydemus as prominent interlocutors. According to the dialogue, the brothers were born on the Aegean island of Chios before relocating as colonists to Thurii in Magna Graecia of modern-day Italy. After being exiled from Thurii, perhaps in 413, they came to Athens. According to Socrates in the Euthydemus, the two taught fighting in armor and legal oration before developing an interest in sophism. Xenophon in the Memorabilia further attributes the teaching of generalship to Dionysodorus specifically.
Additionally, an individual named Dionysodorus appears in Lysias' Against Agoratus speech, who potentially matches the sophist on several biographical details. This Dionysodorus was a general and Taxiarch who supported the democracy; if the general and sophist are one and the same, Dionysodorus may have become a naturalized Athenian citizen along with many other foreign residents before the Battle of Arginusae.
Throughout the Euthydemus, Plato depicts Dionysodorus and his brother employing a string of logical fallacies to defeat Socrates via his student Clinias (III), son of Axiochus. In doing so, scholars have suggested that Plato here chose the brothers as token sophists worthy of ridicule. Aristotle preserves, and refutes, a specific argument of Euthydemus which held that "a man knows that there is a trireme in the Piraeus because he knows each of the two things ['a trireme' and 'in the Piraeus'] separately."
In Xenophon Socrates examines a student of Dionysodorus, from whom it appears Dionysodorus has not taught some of the basic elements of generalship. The implication seems to be either that Dionysodorus has shamelessly taken the student's payments, without giving him his money worth, or that Dionysodorus himself is ignorant of the very art he claims to teach, specifically generalship. This is in keeping with the negative light cast upon Dionysodorus by Plato, although the biographical details are in conflict.