Onomastus is a genus of jumping spiders.
Onomastus may also refer to:
Onomastus of Macedon (fl. 185 BC), an ancient Greek officer Onomastus of Smyrna (fl. 688 BC), an ancient Greek boxer Onomastus of Elis (fl. 6th century BC), son of Agaeus, according to Herodotus, one of the many Greek suitors who came to court Agariste of Sicyon