In Greek mythology, Io (; Ancient Greek: Ἰώ [iːɔ̌ː]) was one of the mortal lovers of Zeus. An Argive princess, she was an ancestor of many kings and heroes, such as Perseus, Cadmus, Heracles, Minos, Lynceus, Cepheus, and Danaus. The astronomer Simon Marius named a moon of Jupiter after Io in 1614.

Because her brother was Phoroneus, Io is also known as Phoronis (an adjective form of Phoroneus: "Phoronean"). She was sometimes compared to the Egyptian goddess Isis, whereas her Egyptian husband Telegonus was Osiris.