In Greek mythology, Eumolpus was the son of Poseidon and Chione (or Hermes and Aglaulus).

According to Apollodorus (3.15.4) Chione daughter of Boreas and Orithyia, pregnant with Eumolpus by Posiedon, was frightened of her father's reaction so she threw the baby into the ocean. Poseidon looked after him and brought him to shore in Ethiopia, where Benthesikyme, a daughter of Poseidon and Amphitrite, raised the child, who then married one of Benthesikyme's two daughters by her Ethiopian husband. Eumolpus however loved a different daughter and Benthesikyme sent him in exile to Thrace. There, he helped plan a plot to overthrow King Tegyrios (who later forgave him on his deathbed and bequeathed his kingdom to Eumolpus). Eumolpus then fled to Eleusis.

In Eleusis, Eumolpus became on the first priests of Demeter and one of the founders of the Eleusinian Mysteries.

Eumolpus was an excellent musician and singer; he played the aulos and the lyre. He won a musical contest in the funereal games of Pelias. He taught music to Heracles.

During a war between Athens and Eleusis, Eumolpus sided with Eleusis. His son, Himmarados, was killed by King Erechtheus. In some sources, Erechtheus also killed Eumolpus and that Poseidon asked Zeus to avenge his son's death. Zeus killed Erechtheus with a lightning bolt or Poseidon made the earth open up and swallow Erechtheus.

Eleusis lost the battle with Athens but the Eumolpides and Kerykes, two families of priests to Demeter, continued the Eleusinian mysteries. Eumolpus' youngest son, Herald-Keryx founded the lines.

Alternative: Eumolpos

Apollodorus : II,5,12 and III,15,1, 4; Homeric Hymn to Demeter 147, 474.