Proteus (not to be confused with Proëtus) was an ancient sea god, a so-called Old Man of the Sea (halios geron), who was occasionally described as a son of Poseidon or Nereus (the latter according to Euripides).

His name would appear to mean simply 'first' (protos) in any appropriate sense, such as 'first to rule', 'first-born', 'first in time' and the like. Thomas Keightley suggests that the name should really be 'Ploteus' and so mean 'floater', 'swimmer', from plotos, 'sailing', 'floating', 'swimming'.