In Greek mythology and legendary history, Callithyia (/ˌkælᵻˈθaɪ.ə/; Greek: Καλλίθυια; also Callithoe (/ˌkəˈlɪθoʊiː/; Καλλιθόη), Callithea (/kəˈlɪθiə/; Καλλιθέα), or Io (/ˈaɪ.oʊ/; Ἰώ [iːɔ̌ː])), "the best among women as well as among men", was the daughter of Peiras or Peiranthus (himself son of Argus) and the first priestess of Argive Hera in history. Her father was credited with founding the first temple of Hera in Argolis, as well as with carving a wooden image of the goddess for the sanctuary; it was at this temple that Callithyia performed her duties as priestess. Scholia on Aratus mention her as the inventor of the chariot and the mother of Trochilus.
Callithyia is perhaps identical with "Io Callithyessa", "the first priestess of Athena" according to Hesychius of Alexandria. In a lesser known version of the Argive genealogy, Io was the daughter of Peiren, likely the same as Peiras.