(antipope Sept. 366–Nov. 367: d. 385?)
Nothing is known of his earlier history except that he was one of Liberius' deacons. On Liberius' death on 24 Sept. 366 the animosity between his supporters and those of Antipope Felix II, dormant since the latter's death on 22 Nov. 365, erupted afresh. The dead pope's unwavering adherents, including priests and three deacons, immediately assembled in the Julian basilica (Sta Maria in Trastevere), elected Ursinus, and had him consecrated there and then by Bishop Paul of Tibur (Tivoli). They were probably opponents of the irenic policy of Liberius, who had done his best to heal the schism after Felix's death. The former partisans of Felix, however, who may have been more numerous, elected the deacon Damasus, who thereupon hired a mob, savagely attacked the Ursinians, and was himself consecrated pope on 1 Oct. The bloody street-fighting continued, but Damasus eventually got the upper hand, enlisting the help of the city prefect, Viventius, who sent Ursinus and his deacons, Amantius and Lupus, into exile. He also had his priests arrested, but the Ursinians rescued them and then established themselves in the Liberian basilica (Sta Maria Maggiore), which they used for a while for worship. In response to their appeal, Emperor Valentinian I (364–75) now ordered the new city prefect, Vettius Agorius Praetextatus, to permit the return of Ursinus and his colleagues on condition that they kept the peace. The antipope and his deacons re-entered the city in triumph on 15 Sept. 367. There were renewed disorders, however, and Damasus (it was alleged) having bribed the court, Ursinus was again exiled on 16 Nov., this time to Gaul. His clergy and many of his supporters were also expelled, and their last remaining church was, on government orders, handed over to Damasus. The Ursinians, however, continued to meet without clergy in cemeteries and in the church of Sta Agnese on the Via Nomentana, although they were soon brutally dislodged by the pope's henchmen. The bishops of Italy were understandably upset by the reports they received, and at a Roman synod (autumn 368) to celebrate Damasus' birthday pointedly turned down his request that they should condemn Ursinus. The government nevertheless maintained active support for Damasus, and c.370 issued instructions that the Ursinians should not hold meetings nearer than the twentieth milestone from Rome. A precarious peace having been thus patched up, Ursinus and leading members of his faction were released from confinement on the understanding that they did not set foot in Rome or its outskirts. They now settled in north Italy, and in the early 370s used a converted Jew named Isaac to bring a ‘disgraceful charge’ (of adultery, according to LP) against the pope. The charge was dismissed but the civil power had no option but to intervene again, relegating Ursinus to Cologne and Isaac to Spain; the Ursinians were forbidden to come within a hundred miles of Rome. Even so, the sentence seems to have been relaxed, for in Sept. 381 the synod of Aquileia complained to the emperors that Ursinus was still fomenting mischief, and requested them to get rid once for all of so persistent a troublemaker. Yet he never abandoned hope of attaining the papal throne, and put himself forward as a candidate on Damasus' death in Dec. 384. An imperial letter survives, dated 24 Feb. 385, expressing the court's relief that he had been howled down and that Siricius had been decisively elected. The date of his death is not known.