(Ὀνόμαρχος), general of the Phocians in the Sacred War, was brother of Philomeins and son of Theotimnus (Diod. 16.56, 61 ; Paus. 10.2.2; but see Arist. Pol. 5.4, and Thirlwall's Greece, vol. v. p. 275, not.). He commanded a division of the Phocian army under Philomelus, in the action at Tithorea, in which the latter perished; and after the battle gathered together the remains of the Phocian army, with which he effected his retreat to Delphi. An assembly of the people was now held, in which Onomarchus strongly urged the prosecution of the war, in opposition to the counsels of the more moderate party, and succeeded in obtaining his own nomination to the chief command in the place of Philomelus, B. C. 353. He was, however, far from imitating the moderation of his predecessor: he confiscated the property of all those who were opposed to him, and squandered without scruple the sacred treasures of Delphi. The latter enabled him not only to assemble and maintain a large body of mercenary troops, but to spend large sums in bribing many of the leading persons in the hostile states; by which means he succeeded in prevailing on the Thessalians to abandon their allies, and take up a neutral position. Thus freed from his most formidable antagonists, he was more than a match for his remaining foes. He now invaded Locris, took the town of Thronium, and compelled that of Amphissa to submit; ravaged the Dorian Tetrapolis, and then turned his arms against Boeotia, where he took Orchomenus and laid siege to Chaeroneia, but was compelled to retreat without effecting anything more. His assistance was now requested by Lycophron, tyrant of Pherae, who was attacked by Philip, king of Macedonia ; and he at first sent his brother Phayllus into Thessaly with an army of 7000 men. But Phayllus having been defeated by Philip, Onomarchus marched with his whole forces to the support of Lycophron, defeated Philip in two successive battles, and drove him out of Thessaly. He next turned his arms a second time against the Boeotians, whom he defeated in a battle, and took the city of Coroneia, when he was recalled once more to the assistance of Lycophron, against Philip, who had again invaded Thessaly. Onomarchus hastened to support his ally with an army of 20.000 foot and 500 horse, but was met by Philip at the head of a force still more numerous, and a pitched battle ensued, in which the superiority of the Thessalian cavalry decided the victory in favour of the king. Onomarchus himself, with many of the fugitives, plunged into the sea in hopes to reach by swimming the Athenian ships under Chares, which were lying off the shore, but perished in the waves, or, according to Pausanias, by the darts of his own soldiers. His body fell into the hands of Philip, who caused it to be crucified, as a punishment for his sacrilege. His death took place in B. C. 352 (Diod. 16.31_33, 35, 56, 61; Paus. 10.2.5 ; Just. 8.1, 2; Polyaen. 2.38; Ephorus, fr. 153, ed. Didot; Oros. 3.12; Wesseling, ad Diod. 16.35; Dem. de Fals. Leg. p. 443). We are told that Onomarchus was a man of luxurious habits, and that he made use of the sacred treasures, not only for the purposes of the state, but to minister to his own pleasures (Theopomp. apud Athen. xiii. p. 605); but it is difficult to know what value to attach to such statements; the religious character assumed by the enemies of the Phocians having led them to load with obloquy the memory of all the leaders of that people.