Deiotarus was a tetrarch of Galatia (Gallo-Graecia) in Asia Minor, and a faithful ally of the Romans.

He is first heard of at the beginning of the third Mithradatic war , when he drove out the troops of Mithradates under Eumachus from Phrygia. His most influential friend was Pompey, who, when settling the affairs of Asia (63 or 62 B.C.), rewarded him with the title of king and an increase of territory (Lesser Armenia). On the outbreak of the civil war, Deiotarus naturally sided with his old patron Pompey, and after the battle of Pharsalus escaped with him to Asia.

In the meantime Pharnaces , the son of Mithradates, had seized Lesser Armenia, and defeated Deiotarus near Nicopolis. Fortunately for Deiotarus, Julius Caesar at that time (47) arrived in Asia from Egypt, and was met by the tetrarch in the dress of a suppliant. Caesar pardoned him for having sided with Pompey, ordered him to resume his royal attire, and hastened against Pharnaces, whom he defeated at Zela.

In consequence of the complaints of certain Galatian princes, Deiotarus was deprived of part of his dominions, but allowed to retain the title of king. On the death of Mithradates of Pergamum , tetrarch of the Trocmi, Deiotarus was a candidate for the vacancy. Other tetrarchs also pressed their claims; and, further, Deiotarus was accused by his grandson Castor of having attempted to assassinate Caesar when the latter was his guest in Galatia. Cicero, who entertained a high opinion of Deiotarus, whose acquaintance he had made when governor of Cilicia, undertook his defence, the case being heard in Caesars own house at Rome. The matter was allowed to drop for a time, and the assassination of Caesar prevented any final decision being pronounced. In his speech Cicero briefly dismisses the charge of assassination, the main question being the distribution of the provinces, which was the real cause of the quarrels between Delotarus and his relatives. After Caesars death, Mark Antony, for a large monetary consideration, publicly announced that, in accordance with instructions left by Caesar, Deiotarus was to resume possession of all the territory of which he had been deprived. When civil war again broke out, Deiotarus was persuaded to support Brutus and Cassius, but after the battle of Philippi went over to the triumvirs. He remained in possession of his kingdom till his death at a very advanced age.

References

• Cicero, Philippica, ii. 37; Ad Jam. viii. 10, ix. 12, xv. I, 2, 4; Ad A/I. xiv. 1; De diem. i. 15, ii. 36, 37; De harusp. resp. 13, and above all Pro 1ege Deiotaro
• Appian, Bell. Milhrid. 75, 114; Bellum Alexandrinum, 34-41, 65-77
• Dio Cassius xli. 63, xlii. 45, xlvii. 24, 48, xlviii. 33.