Timaeus (/taɪˈmiːəs/; Ancient Greek: Τίμαιος, romanized: Timaios, pronounced [tǐːmai̯os]) is one of Plato's dialogues, mostly in the form of long monologues given by Critias and Timaeus, written c. 360 BC. The work puts forward reasoning on the possible nature of the physical world and human beings and is followed by the dialogue Critias.

Participants in the dialogue include Socrates, Timaeus, Hermocrates, and Critias. Some scholars believe that it is not the Critias of the Thirty Tyrants who appears in this dialogue, but his grandfather, also named Critias. At the beginning of the dialogue, the absence of another, unknown dialogue participant, present on the day before, is bemoaned. It has been suggested from some traditions—Diogenes Laertius (VIII 85) from Hermippus of Smyrna (3rd century BC) and Timon of Phlius (c. 320 – c. 235 BC)—that Timaeus was influenced by a book about Pythagoras, written by Philolaus, although this assertion is generally considered false.

The dialogue takes place the day after Socrates described his ideal state. In Plato's works, such a discussion occurs in the Republic. Socrates feels that his description of the ideal state was not sufficient for the purposes of entertainment and that "I would be glad to hear some account of it engaging in transactions with other states" (19b).

Hermocrates wishes to oblige Socrates and mentions that Critias knows just the account (20b) to do so. Critias proceeds to tell the story of Solon's journey to Egypt where he hears the story of Atlantis, and how Athens used to be an ideal state that subsequently waged war against Atlantis (25a). Critias believes that he is getting ahead of himself, and mentions that Timaeus will tell part of the account from the origin of the universe to man.

Critias also cites the Egyptian priest in Sais about long-term factors on the fate of mankind:

There have been, and will be again, many destructions of mankind arising out of many causes; the greatest have been brought about by the agencies of fire and water, and other lesser ones by innumerable other causes. There is a story that even you [Greeks] have preserved, that once upon a time, Phaethon, the son of Helios, having yoked the steeds in his father's chariot, because he was not able to drive them in the path of his father, burnt up all that was upon the earth, and was himself destroyed by a thunderbolt. Now this has the form of a myth, but really signifies a declination of the bodies moving in the heavens around the earth, and a great conflagration of things upon the earth, which recurs after long intervals.

The history of Atlantis is postponed to Critias. The main content of the dialogue, the exposition by Timaeus, follows.

Timaeus begins with a distinction between the physical world, and the eternal world. The physical one is the world which changes and perishes: therefore it is the object of opinion and unreasoned sensation. The eternal one never changes: therefore it is apprehended by reason (28a).