For the click beetle genus, see Megapenthes .

In Greek mythology, Megapénthês was a son of Proetus. He exchanged kingdoms (Argos for Tiryns) with his cousin Perseus, whom he killed much later. He was the father of Argeus and possibly Anaxagoras.

Also in Greek mythology, Megapénthês was a son of Menelaus, who ruled after his father's death and sent Helen into exile. In some legends, Helen was his mother; in most, however, he was Menelaus's son by a concubine and was born during the Trojan War. His name means "great sorrow" and it is likely a reference to the unhappiness of Menelaus and Helen, even though the Trojan War was won and the royal couple had come back home safely.

Megapenthês is mentioned in the Odyssey at IV.11.