Protagoras is a dialogue by Plato. The traditional subtitle (which may or may not be Plato's) is "or the Sophists". 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 the nature of Sophists, the unity and the teachability of virtue. A total of twenty-one people are named as present.

Of the twenty-one people who are specifically said to be present, three are known Sophists. In addition to Protagoras himself, there are Hippias of Elis and Prodicus of Ceos. Two of the sons of Pericles are said to be there, Paralus and Xanthippus. With the exception of Aristophanes, all of Socrates' named friends from the Symposium are in attendance: Eryximachus the doctor, and Phaedrus are there, and so are the lovers Pausanias and Agathon (who is said to be a mere boy at this point), and Alcibiades. Additionally, there are several unnamed foreigners whom Protagoras is said to have picked up in his travels and a servant (a eunuch) in the employ of Callias.

The dialogue begins with an unnamed friend of Socrates asking him how his pursuit of the young Alcibiades, just now reputed to be growing his first beard, was proceeding. Socrates explains that while he has just been in the company of Alcibiades, his mind is now on more interesting matters. He says that the Sophist Protagoras, the wisest man alive (309c–d), is in town. Socrates relates the story of how his young friend, Hippocrates, son of Apollodorus, came knocking on his door before daybreak and roused him out of bed. Hippocrates was in a big hurry to be present when Protagoras held court, as he was expected to do, at the home of Callias, and wanted Socrates to introduce him as a potential student to the old Sophist, as Protagoras had a great reputation as a teacher.