Onomacritus (c. 530 - 480 BCE) was a Greek chresmologue, or compiler of oracles, who lived in Athens.

Herodotus reports that Onomacritus was hired by the tyrant Pisistratus to compile the oracles of Musaeus, but that Onomacritus inserted forgeries of his own that were detected by Lasus of Hermione. Later on in Herodotus's narrative, the Pisistratids use Onomacritus's misleading prophecies to flatter the King of Persia.

Pausanias attributes certain forged poems in Orpheus's name to Onomacritus.