(Προυσίας), king of Bithynia, was the son of Zielas, whom he succeeded on the throne, and grandson of NICOMEDES I. The date of his accession is unknown, but it appears that it preceded the death of Antiochus Hierax, and may therefore be placed at least as early as B. C. 228, (Trog. Pomp. Prol. xxvii.; Clinton, F. H. vol. iii. pp. 413, 414; Niebuhr, Kl. Schrift. p. 287.) The first event of his reign, which is recorded to us, is a war with the Byzantines, in which we find him engaging in B. C. 220, in conjunction with the Rhodians. The latter were at first supported by Attalus, king of Pergamus, as well as by Achaeus, who had lately assumed the sovereignty of Asia Minor, and they endeavoured also to set up Tiboetes, the uncle of Prusias, as a competitor for the throne of Bithynia. Their efforts were, however, unsuccessful: Prusias conquered all the possessions of the Byzantines in Asia, while the Thracians pressed them closely on the European side, and they were soon compelled to submit to a peace on disadvantageous terms. (Plb. 4.47_52.) Shortly after this, in B. C. 217, Prusias is mentioned among the princes who sent costly presents to the Rhodians after the great calamity they had suffered by an earthquake: and the following year (216) he obtained great distinction by defeating and cutting to pieces a formidable army of Gauls, who had been invited into Asia by Attalus, and had become the terror of the adjoining countries. (Id. 5.90, 111.) On the breaking out of the war between the Romans and Philip, king of Macedon, Prusias lent his assistance to the latter ; and besides supplying him with an auxiliary squadron of ships, rendered him a more important service by invading the territories of his own neighbour and rival Attalus, whom he thus recalled from Greece to the defence of his own kingdom, B. C. 207. (Liv. 27.30, 28.7.) The name of the Bithynian monarch was, in consequence, included in the treaty of peace between Philip and the Romans in B. C. 205 (Liv. 29.12), and we subsequently find the two kings uniting their forces to besiege Cius in Bithynia, which, after it had fallen into their hands, was sacked by order of Philip, the inhabitants sold as slaves, and the city itself given up to Prusias. (Plb. 15.21, 17.5; Liv. 32.34; Strab. xii. p.563.)

It does not appear that the latter, though he was connected by marriage with the Macedonian king, took any part in the decisive struggle of Philip with the Roman power (B. C. 200-196): but in B. C. 190, when Antiochus was, in his turn, preparing to contend with the republic, he made repeated attempts to obtain the alliance of Prusias, who was at first disposed to listen to his overtures, but yielded to the arguments of the two Scipios, and concluded an alliance with Rome, though he appears to have, in fact, taken no part in the war that followed. (Plb. 21.9; Liv. 37.25 ; Appian. Syr. 23.) After the termination of that war, however, Prusias became involved in hostilities with Eumenes, king of Pergamus, by which he gave umbrage to the Romans, and he soon after greatly increased this offence by affording a shelter to their implacable enemy, the fugitive Hannibal. The exiled general rendered important services to the king in his contest with Eumenes, but, notwithstanding these obligations, Prusias was unwilling to brave the anger of Rome, and when Flamininus was deputed by the senate to demand the surrender of Hannibal, the king basely gave his consent, and the Carthaginian general only escaped falling into the hands of his enemies by a voluntary death. (Plb. 23.18, 24.1; Liv. 39.51; Justin, 32.4; Plut. Flam. 20 ; Corn. Nep. Hann. 10-12; App. Syr. 11; Eutrop. 4.5; Oros. 4.20; Strab. xii. p.563.)

This is the last circumstance which can be referred with certainty to the elder Prusias: the period of his death, and of the accession of his son, is not mentioned by any ancient writer, but Mr. Clinton regards the Prusias mentioned in the treaty of B. C. 179, between Eumenes and Pharnaces, as the second king of this name: and this supposition, though not admitting of proof, appears at least a very probable one. (Clinton, F. H. vol. ii. p. 417.) In this case we must place his death between 183 and 179 B. C. It was apparently during the latter part of his reign that Prusias, who had already made himself master of Cierus, Tieios, and other dependencies of Heracleia, laid siege to that city itself; but while pressing the attack with vigour, he himself received a severe wound from a stone, which not only compelled him for a time to abandon the enterprise, but left him with a lameness for the remainder of his life. On this account he is sometimes distinguished by the epithet of the Lame (ὁ χωλος) (Memnon. 100.27, ed. Orell.)

Prusias appears to have been a monarch of vigour and ability, and raised his kingdom of Bithynia to a much higher pitch of power and prosperity than it had previously attained. Like many of his contemporary princes, he sought distinction by the foundation or new settlement of cities, among the most conspicuous of which were Cius and Myrleia on the Propontis, which he repeopled and restored after their ruin by Philip, bestowing on the one his own name, while he called the other after his wife, Apameia. In addition to this, he gave the name of Prusias also to the small city of Cierus, which he had wrested from the Heracleians. (Strab. xii. p.563; Steph. Byz. s. v. Προῦσα and Ἀπάμεια, Memnon. 100.41. 47.) The foundation of Prusa, at the foot of Mount Olympus, is also ascribed to him by some authors. (Plin. Nat. 5.43. See on this point Droysen, Hellenism. vol. ii. p. 655.) Before the close of his reign, however, his power received a severe blow by the loss of the Hellespontine Phrygia, which he was compelled to cede to the kings of Pergamus; probably by the treaty which terminated the war already alluded to. (Strab. l.c.)