In Greek mythology, Proteus is an early prophetic sea god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea". Some who ascribe a specific domain to Proteus call him the god of "elusive sea change", which suggests the changeable nature of the sea or the liquid quality of water. He can foretell the future, but, in a mytheme familiar to several cultures, will change his shape to avoid doing so; he answers only to those who are capable of capturing him. From this feature of Proteus comes the adjective protean, meaning "versatile", "mutable", or "capable of assuming many forms". "Protean" has positive connotations of flexibility, versatility and adaptability.

In Greek mythology, the Old Man of the Sea was a figure who could be identified as any of several water-gods, generally Nereus or Proteus, but also Triton, Pontus, Phorcys or Glaucus. He is the father of Thetis.