Glaucus was a Greek sea-god, the son of Anthedon and Alcyone. The story of his origin is told by Ovid, that he began as a mortal fisherman living in the Boeotian city of Anthedon and one day he caught and landed some fish at a place where there grew a herb with the magic property of resuscitating fish and allowing them to return to the water. Seeing this effect the herb had on the fish, Glaucus ate some of it too. The herb made him immortal, but it also gave him fins and caused his legs to transform into a fish's tail, forcing him to dwell forever in the sea. Glaucus was initially upset by this side-effect, but Oceanus and Tethys received him well and he was quickly accepted among the dieties of the sea, learning the art of prophecy at which they were skilled.
Glaucus later fell in love with the Italian maiden Scylla, who rejected him due to his piscine form. He consulted with Circe for a solution but she became passionately in love with him herself. Since Glaucus cared only for Scylla, however, Circe turned her into a fishlike monster from the waist down, with a row of vicious dog's heads round her loins. She went to live alone in a submerged cave on the Straits of Messina, but Glaucus remained in love with her and mourned her transformation.
Euripides wrote in his play Orestes that Glaucus was a son of Nerus and says that he assisted Menelaus on his homeward journey with good advice. He also helped the Argonauts. It was believed that he commonly came to the rescue of sailors in storms, having once been one himself.