Prosymnus or Polymnus (Greek: Πρόσυμνος/Πόλυμνος), in Greek mythology, was a shepherd living near the reputedly bottomless Alcyonian Lake, hazardous to swimmers, which lay in the Argolid, on the coast of the Gulf of Argos, near the prehistoric site of Lerna. This story is not told in full by any of the usual sources of Greek mythological tales, though several of them hint at it. It is reconstructed on the basis of statements by Christian authors; these have to be treated with reserve because their aim is to discredit pagan mythology.