Orestheus (Ancient Greek: Ὀρεσθεύς derived from oresteros "mountainous" from Greek όρος oros "mountain, hill"), in Greek mythology, was a name attributed to two individuals.
- Orestheus, a son of Deucalion and Pyrrha, was king of the Ozolian Locrians in Aetolia. His dog was said to have given birth to a piece of wood, which Orestheus concealed in the earth. In the spring, a vine grew forth from it, from the sprouts of which (Greek ὅζοι ozoi "branches") he derived the name of his people.
- Orestheus, a son of Lycaon, was king of Arcadia and the reputed founder of Arcadian Oresthasion, which is said afterwards to have been called Oresteion, from Orestes.