The Theætetus (Greek: Θεαίτητος) is one of Plato's dialogues concerning the nature of knowledge. The framing of the dialogue begins when Euclides tells his friend Terpsion that he had written a book many years ago based on what Socrates had told him of a conversation he'd had with Theaetetus when [Theaetetus] was quite a young man. (Euclides also notes that he'd had to go back to Socrates to ask some more questions about the speeches due to his spotty recollection of the account.)

Euclides is prompted to share his book when Terpsion wonders where he'd been: Euclides, who apparently can usually be found in the marketplace of Megara, was walking outside of the city and had happened upon Theaetetus being carried from Corinth to Athens with a case of dysentery and a minor war wound; Euclides remarks that Socrates had made some uncanny predictions about Theaetetus needing to rise to fame. Euclides' book is read aloud to the two men by a slave boy in the employ of Euclides.

In this dialogue, Socrates and Theaetetus discuss three definitions of knowledge: knowledge as nothing but perception, knowledge as true judgment, and, finally, knowledge as a true judgment with an account. Each of these definitions are shown to be unsatisfactory. The conversation ends with Socrates' announcement that he has to go to court to answer to the charges that he has been corrupting the young and failing to worship Athenian Gods.