(Ὀνόμαστος,) a confidential officer of Philip V. of Macedon, for whom he held the government of the sea-coast of Thrace, and whose instrument he was, together with CASSANDER [No. 4], in the massacre of the Maronites. Appius Claudius, and the other Roman colnmissioners, required that Philip should send Onomastus and Cassander to Rome to be examined about the massacre; whereupon the king despatched Cassander, and had him poisoned on the way, but persisted in declaring that Onomastus had not been in or near Maroneia at the time; the fact being (as Polybius and Livy tell us) that he was too deep in the royal secrets to be trusted at Rome. We hear again of Onomastus as one of the two assessors of Philip at the private trial of DEMETRIUS for the alleged attempt on the life of his brother Perseus, B. C. 182. (Plb. 23.13, 14 ; Liv. 39.34. 40.8.)