- Euclid of Megara - the founder of the Megarian school of philosophy, a student of Socrates and contemporary of Plato.[2]
- Terpsion - a friend of Euclid who is unknown outside of this dialogue, although later fables about him, that were likely written on the basis of this dialogue are preserved by Plutarch[3] and in the Cynic epistles.[4]
- Socrates - the famous teacher of Plato, who was executed by the people of Athens in 399 BC. In the dialogue, which takes place just before Socrates visits the Palace of the King Archon for his Trial of Socrates, Socrates is an old man of about 70.
- Theodorus - a Greek mathematician from Cyrene, a prosperous Greek colony on the coast of North Africa, in what is now Libya, on the eastern end of the Gulf of Sidra. Theodorus explored the theory of incommensurable quantities, and according to Diogenes Laertius, was said to have taught mathematics to Plato, although the historicity of this claim cannot be verified.[5]
- Theaetetus - A Greek mathematician from Athens, who is credited in Book X of Euclid's Elements with developing a method for measuring irrational lengths in terms of square roots, as well as the proof that there are precisely five regular convex polyhedra.[6] According to the dialogue, he evidently resembled Socrates in the snubness of his nose and bulging of his eyes. He apparently died from wounds and dysentery after the battle in Corinth that occurs in the frame story of the dialogue.
Socrates, Theaetetus, and Theodorus reappear the following day in the Sophist an apparent continuation of the conversation contained within the book of Euclid, where they are also joined by an unnamed Eleatic stranger and a boy also named Socrates.
