Critias (/ˈkrɪtiəs/; Greek: Κριτίας), one of Plato's late dialogues, recounts the story of the mighty island kingdom Atlantis and its attempt to conquer Athens, which failed due to the ordered society of the Athenians. Critias is the second of a projected trilogy of dialogues, preceded by Timaeus and followed by Hermocrates. The latter was possibly never written and the ending to Critias has been lost. Because of their resemblance (e.g., in terms of persons appearing), modern classicists occasionally combine both Timaeus and Critias as Timaeus-Critias.

Unlike the other speakers of the Critias, it is unclear whether Timaeus is a historical figure or not. While some classicists regard him as definitively historical, others guess that "Plato's picture of him has probably borrowed traits from various quarters". Frank assumes Archytas of Tarentum to be the person which Timaeus is partly based on.

On the other hand, F. M. Cornford strongly opposes any idea of a historical Timaeus: "The very fact that a man of such distinction left not the faintest trace in political or philosophic history is against his claim to be a historical person. The probability is that Plato invented him because he required a philosopher of the Western School, eminent both in science and statesmanship, and there was no one to fill the post at the imaginary time of the dialogue". But while there is no proof for Timaeus to be historical, there is also no proof that he did not exist, since little is known of the history of the Italian city of Locri.

From the very first comments on Timaeus and Critias in classical antiquity to the early 20th century, scholars took the identity of this Critias and the oligarch Critias for granted. The first to contradict this view was Burnet in 1914. Since then, the identity of Critias has been fiercely disputed among scholars. One group of classicists still claims him to be the famous oligarch Critias, member of the Thirty Tyrants. Another suggests that this Critias is actually the grandfather of the oligarch.

The latter group argues that there is too much distance of time between the oligarch Critias (460–403 BC) and Solon (638–558 BC), the famous lawmaker, who supposedly brought the Atlantis story from Egypt to Greece. According to Plato, Solon told the story to the great-grandfather of the Critias appearing in this dialogue, Dropides, who then told it to his son, who was also named Critias and the grandfather of the Critias in the dialogue. The elder Critias then retold the story to his grandson when he was 90 and the younger Critias was 10.

The latter group alleges that the tyrant's grandfather could not have both talked to Solon and still have been alive at the time the hypothetical discussion pictured in this dialogue was held. Thus they assume that it is the tyrant's grandfather who appears in both Timaeus and Critias, and his own grandfather, who was told the Atlantis story by Solon.

On the other hand, this obviously too long time span between Solon and Critias would not be the only anachronism in Plato's work. In fact, Plato produced quite a number of anachronisms in many of his dialogues. And further, there are indications that Solon was dated later than when he actually lived by writers prior to Aristotle.

This leads one to believe that Plato somewhat condensed the happenings of the sixth century. For his purposes, Solon lived just before Anacreon, and Anacreon in turn was active in the early fifth century. The elder Critias is not known to have achieved any personal distinction, and since he died long before Plato published the Timaeus and Critias, it would have made no sense for Plato to choose a virtually unknown statesman to appear in these dialogues who was uninteresting to his contemporaries.