Thrasyalces was a sophist, a type of professional teacher, active in Greece during the 5th century BCE. He was from the island of Thasos. His existence is known from a single brief mention in Plato’s dialogue Theaetetus, where Socrates names him as a contemporary of the more famous sophist Protagoras. No other details of his life or career are recorded.

No writings or specific doctrines by Thrasyalces survive, and none are mentioned in ancient sources. His significance is entirely as a historical footnote. According to modern scholars, his sole appearance in Plato’s text serves to highlight the practices of the better-known Protagoras. Thrasyalces represents the many minor sophists who were part of the intellectual landscape of classical Athens, against whom philosophers like Socrates and Plato often defined their own ideas. He is a reminder of the wide range of educators active at the time, even if his own work is completely lost.