in Greek mythology, the prophetic old man of the sea and shepherd of the sea's flocks (e.g., seals). He was subject to the sea god Poseidon, and his dwelling place was either the island of Pharos, near the mouth of the Nile River, or the island of Carpathus, between Crete and Rhodes.

Proteus knew all things—past, present, and future—but disliked telling what he knew. Those who wished to consult him had first to surprise and bind him during his noonday slumber. Even when caught he would try to escape by assuming all sorts of shapes. But if his captor held him fast, the god at last returned to his proper shape, gave the wished-for answer, and plunged into the sea. The captor in Homer's version (Odyssey, Book IV) was Menelaus; in Virgil's telling (Georgics, Book IV), it was Aristaeus who tried to hold Proteus. Because Proteus could assume whatever shape he pleased, he came to be regarded by some as a symbol of the original matter from which the world was created.

In a story first known from the work of the 6th-century-BC poet Stesichorus, Proteus was portrayed as an Egyptian king—either of Memphis (by Herodotus) or of all Egypt (in Euripides' Helen)—who kept the real Helen safe in Egypt while Zeus sent Paris on his way to Troy with a phantom Helen.