Protagoras (in Greek Πρωταγόρας) was born around 481 BC in Abdera in Ancient Greece. He was a pre-Socratic philosopher and is numbered as one of the sophists by Plato, who in his dialogue of the same name credits him with having invented the role of the professional sophist or teacher of "virtue". He died c. 420 BC. Protagoras was famous as a teacher of rhetoric and debate which were vital to Greek social life. Due to those interests, he was fascinated by the study of orthoepeia, or the correct use of words. His most famous saying is: "Man is the measure of all things, of those that are that they are, and of those that are not that they are not." Despite its fame, this phrase was passed down absent any context, and its meaning isn't entirely clear. It is generally thought to be promoting relativism. It was his teachings that spurred later philosophers such as Plato to search for objective, transcendent guidelines to underly moral behavior, and the importance of subjectivity is an important theme in modern philosophy. Protagoras was also a famous proponent of agnosticism. In "On the Gods," he wrote, "Concerning the gods, I have no means of knowing whether they exist or not or of what sort they may be, because of the obscurity of the subject, and the brevity of human life."

The Protagoras crater on the Moon was named in his honor.

Protagoras is also the title of a dialogue by Plato. See Protagoras (dialogue).