Iasius (Ἰάσιος) is the name of six characters in Greek mythology.
- Iasius, another form of Iasion's name.
- Iasius, one of the Dactyls at Crete.
- Iasius (also Iasion; Iasus), son of Lycurgus, king of Arcadia by Cleophyle or Eurynome. He became king of Arcadia and was the father of Atalanta by Clymene, daughter of Minyas.
- Iasius, son of Eleuther (son of Apollo and Aithousa, daughter of Poseidon) and father of Chairesilaos.