Neocaesarea is a Catholic titular see. The original diocese was in Pontus Polemoniacus, at first called Cabira. It corresponds to present-day Niksar, Turkey.
It was one of the favourite residences of Mithridates the Great, who built a palace there, and later of King Polemon and his successors. Pompey made it a city and gave it the name of Diopolis, while Pythodoris widow of Polemon, made it her capital and called it Sebaste. It is not known precisely when it assumed the name of Neocæsarea, mentioned for the first time in Pliny, "Hist. Nat.", VI, III, 1, but judging from its coins, one might suppose that it was during the reign of Tiberius.
In 344 the city was completely destroyed by an earthquake (Hieronymus, "Chron.", anno 2362), meeting a similar fate in 499 (Theodorus Lector, II, 54). During the Middle Ages the Muslims and Christians disputed the possession of Neocæsarea, and in 1068 a Seljuk general, Melik-Ghazi, whose tomb is still visible, captured and pillaged it; later, in 1397, it passed, together with the whole district, under the sway of the Ottomans.
It became the civil and religious metropolis of Pontus. We know that about 240, when Gregory Thaumaturgus was consecrated bishop of his native city, Neocæsarea had only seventeen Christians and that at his death (270) it counted only seventeen pagans. In 315 a great council was held there, the acts of which are still extant.
Being early placed at the head of an ecclesiastical province, Neocæsarea had four suffragan sees about 640 ("Ecthesis" of pseudo-Epiphanius, ed. Heinrich Gelzer, 539), retaining them until the tenth century, when Trebizond obtained its independence and, by degrees, the other three suffragans were suppressed. In 1391 the Archdiocese of Neocæsarea was confided to the metropolitan of Trebizond (Miklosich and Müller, "Acta", II, 154). About 1400 there was, however, a regular metropolitan (op. cit., II, 312), residing at Ordou. Among the twenty-seven bishops of this city mentioned by Le Quien, the most noted are Gregory Thaumaturgus, St. Paul, and St. Thomas, a martyr of the ninth century.