(ca. 490-420 B.C.E.) Ancient Greek Sophist (teacher) of the fifth century B.C.E. whose thinking contributed to the development of dialectic and practical philosophy in general. Protagoras is best remembered for one of two surviving fragments which states "Of all things, the measure is man, of the things that are that they are; and of the things that are not that they are not." Protagoras lived in Abdera.

Protagoras (Greek: ) (c. 490– 420 BC) was a pre-Socratic Greek philosopher and is numbered as one of the sophists by Plato. In his dialogue Protagoras, Plato credits him with having invented the role of the professional sophist or teacher of virtue.

Protagoras is a minor but important dialogue of Plato. The main argument is between the elderly Protagoras, a celebrated sophist, and Socrates. The discussion takes place at the home of Callias, who is host to Protagoras while he is in town, and concerns a familiar theme in the dialogues: the teachability of virtue.

Protagoras is a lunar impact crater that is located on the Mare Frigoris on the northern part of the Moon. To the north-northwest is the slightly larger Archytas crater, and to the southeast is the prominent Aristoteles crater.