In Greek mythology, Ialmenus was a son of Ares and Astyoche, and twin brother of Ascalaphus. Together with his brother, he sailed with the Argonauts, was among the suitors of Helen and led the Orchomenian contingent in the Trojan War.
Unlike Ascalaphus, Ialmenus survived the war. He was said to have ended up in Colchis, where he founded a colony, the inhabitants of which were later referred to as the "Achaeans of Pontus".