Niksar, historically known as Neocaesarea (Νεοκαισάρεια), is a city in Tokat Province, Turkey. It is the seat of Niksar District. Its population is 37,017 (2022). It was settled by many empires. Niksar is known as "Çukurova of North-Anatolia" due to its production of many kinds of fruits and vegetables. On May 2, 2018, Niksar was included in the World Heritage tentative list.

Niksar has been ruled by the Hittite, Persian, Greek, Pontic, Roman, Byzantine, Danishmend, Seljuk and Ottoman Empires. It has always been an important place in Anatolia because of its location, climate and productive farmland.^{[clarification needed]}

It was known as Cabira in the Hellenistic period (Κάβειρα in Greek). It was one of the favourite residences of Mithridates VI, who built a palace there, and later of King Polemon I and his successors.

In 72 or 71 BCE, the Battle of Cabira during the Third Mithridatic War took place at Cabira, and the city passed to the Romans. Niksar was called Diospolis, Sebaste, and Neokaisareia during the Roman period. 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 Neocaesarea, 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 Neocaesarea became part of the Eastern Roman Empire when the Roman Empire divided into two parts in AD 395. Another earthquake occurred in 499, the 499 Nicopolis earthquake.

During the Middle Ages, the Muslims and Christians disputed the possession of Neocaesarea, and in 1068 a Seljuk general, Melik-Ghazi, whose tomb is still visible, captured and pillaged it. When the Seljuqs raided Anatolia in 1067, Neocaesarea was conquered by Afşın Bey, one of the commanders of Alp Arslan. The Byzantines retook the area in 1068. Conquered by Artuk Bey after the Battle of Manzikert, Neocæsarea once again returned to Byzantium in 1073. The city became part of the domain of Roussel de Bailleul, a Norman mercenary who had rebelled against the Byzantine empire, and who held the town until 1075. Melik Gümüştekin Ahmet Gazi (better known as Danishmend Gazi), founder of the Danishmend, was the next conqueror of Neocaesarea. After the conquest the Gazi made it his capital city, and, under the name Niksar, became a center of science and culture. The Danishmend Gazi's mausoleum stands in a large cemetery just outside the town.

In 1100, Bohemond I of Taranto was held prisoner in Niksar castle until 1103.^{[citation needed]}

By 1175, during the reign of Kılıç Arslan II, Niksar was dependent on the Seljuks of Rum. After the Mongol invasion of the 13th century, Niksar was governed by the Eretnids and then the Beylik of Tacettin, a beylik and became the center of the latter principality. After Kadı Burhanettin (who conquered Niksar in 1387) was killed in battle, the people of Niksar sought aid from the Ottoman Sultan Bayezid I. The Sultan's son, Süleyman Çelebi, took Niksar for the Ottomans. In the later Ottoman period, Niksar became part of Tokat Province. Fatih Mehmet launched a raid on Trabzon from Niksar, and Selim I and Suleiman the Magnificent raided the east from there. The town was predominantly Muslim in the mid 17th century.

Neocaesarea was an episcopal see in the late Roman province of Pontus Polemoniacus. At first called Cabira, it became the civil and religious metropolis of Pontus. In around 315, the Synod of Neo-Caesarea was held there. It is now one of the bishoprics listed in the Annuario Pontificio as titular sees and is referred to as Neocaesarea in Ponto to distinguish it from Neocaesarea in Syria.