Alpheus or Alpheios (/ælˈfiːəs/; Ancient Greek: Ἀλφειός, meaning "whitish"), was in Greek mythology a river (the modern Alfeios River) and river-god.
Like most river-gods, he is a son of Oceanus and Tethys. Telegone, daughter of Pharis, bore his son, the king Orsilochus. Through him, Alpheus was the grandfather of Diocles, and great-grandfather of a pair of soldiers, Crethon and Orsilochus, who were slain by Aeneas during the Trojan War.
According to Pausanias, Alpheius was a passionate hunter and fell in love with the nymph Arethusa, but she fled from him to the island of Ortygia near Syracuse, and metamorphosed herself into a well, after which Alpheius became a river, which flowing from the Peloponnese under the sea to Ortygia, there united its waters with those of the well Arethusa. This story is related somewhat differently by the Roman writer Ovid: Arethusa, a beautiful nymph, once while bathing in the river Alfeios in Arcadia, was surprised and pursued by the river god; but the goddess Artemis took pity upon her and changed her into a well, which flowed under the earth to the island of Ortygia.