Promachus was the son of Parthenopaeüs, with another of the name being the son of Aeson by Alcimede (or Polymede), and so the younger brother of Jason. The former Promachus was one of the Sown Men, and was probably killed at Thebes.

The name for both men is a belligerent one, meaning 'champion', literally 'fighting in front', from pro, 'before' and mache, 'battle', 'fight'. For the younger brother of Jason this had to remain a propitious name, since he was killed by Pelias while still a boy.