Roman Armenia refers to the rule of parts of Greater Armenia by the Roman Empire, from the 1st century AD to the end of Late Antiquity. In the late 4th century, Armenia was divided between Rome and the Sasanians, who took control of the larger part of the Armenian Kingdom and in the mid-5th century abolished the Armenian monarchy. In the 6th and 7th centuries, Armenia once again became a battleground between the East Romans (Byzantines) and the Sasanians, until both powers were defeated and replaced by the Muslim Caliphate in the mid-7th century.

In 363, a treaty was signed between the East Roman and Sassanid Persian empires, which divided Armenia between the two. The Persians retained the larger part of Armenia ("Persarmenia") while the Romans received a small part of Western Armenia.

Another treaty followed between 384 and 390, the Peace of Acilisene (usually dated c. 387), which established a definite line of division, running from a point just east of Karin (soon to be renamed Theodosiopolis) to another point southwest of Nisibis in Mesopotamia. The area under East Roman control thus increased, but still, about four fifths of the old Kingdom of Armenia remained under Persian rule.

Unlike Armenia Minor west of the Euphrates, which had been constituted into full provinces (Armenia I and Armenia II) under the Diocese of Pontus already in the time of Diocletian, the new territories retained a varying level of autonomy. Armenia Maior, the northern half, was constituted as a civitas stipendaria under a civil governor titled comes Armeniae, meaning that it retained internal autonomy, but was obliged to pay tribute and provide soldiers for the regular East Roman army.

Under Roman rule, Melitene was the base camp of Legio XII Fulminata. It was a major center in Armenia Minor (P'ok'r Hayk'), remaining so until the end of the 4th century. Emperor Theodosius I divided the region into two provinces: First Armenia (Hayk'), with its capital at Sebasteia (modern Sivas); and Second Armenia, with its capital at Melitene.

The Satrapies (Latin: Gentes) in the south on the other hand, which had been under Roman influence already since 298, were a group of six fully autonomous principalities allied to the Empire (civitates foederatae): Ingilene, Sophene, Antzitene, Asthianene, Sophanene and Balabitene. The local Armenian nakharar were fully sovereign in their territories, and were merely required to provide soldiers upon request and to dispatch a golden crown to the emperor, as a token of submission. In return, they received their royal insignia, including red shoes, from the emperor.

The situation remained unchanged for near a century, until a large-scale revolt by the satraps in 485 against Emperor Zeno (r. 474–491). In its aftermath, the satraps were stripped of their sovereignty and their rights of hereditary succession, being in effect reduced to the status of tax-paying and imperially-administered civitates stipendariae.

Emperor Justinian I (r. 527–565) carried out a series of comprehensive administrative reforms. Already soon after his accession in 527, the dux Armeniae (responsible for Armenia Minor) and the comes Armeniae were abolished, and the military forces of the Armenian territories were subordinated to a new magister militum per Armeniam at Theodosiopolis.

In 536, new reforms were enacted that abolished the autonomy of the trans-Euphrates territories and formed four new regular provinces. Armenia Interior was joined with parts of Pontus Polemoniacus and Armenia I to form a new province, Armenia I Magna, the old Armenia I and Armenia II were re-divided into Armenia II and Armenia III, and the old Satrapies formed the new Armenia IV province. In 538, the Armenian nobles rose up against heavy taxation, but were defeated and forced to find refuge in Persia.

In 591, the treaty between Khosrow II and Maurice ceded most of Persarmenia to the Eastern Roman Empire.

The region was the focus of prolonged warfare in the Byzantine–Sasanian War of 602–628. After the onset of the Muslim conquests and the Arab conquest of Armenia, only the western parts of Armenia remained in Byzantine hands, forming part of the theme of Armeniakon. Armenia remained dominated by the Arabs thereafter, and was ruled by a succession of Caliphate-appointed emirs as well as local princes.

With the ebbing of the Caliphate's power and the fracturing of its outlying territories into autonomous statelets, the Byzantines were able to re-assert their influence over the Armenian principalities during the campaigns of John Kourkouas in the early 10th century. In the first half of the 11th century, under Basil II and his successors, most of Armenia came under direct Byzantine control, which lasted until the Battle of Manzikert in 1071, when all Armenia fell to the Seljuks.

Roman Christianity

The influence of Christianity was felt in the 1st century after Christ: Christianity was first introduced by the apostles Bartholomew and Jude Thaddeus. Thus both Saints are considered the patron saints of the Armenian Apostolic Church.

Apostle Bartholomew is said to have been executed in Albanopolis in Armenia. According to popular hagiography, the apostle was flayed alive and beheaded. According to other accounts he was crucified upside down (head downward) like St. Peter. He is said to have been martyred for having converted Polymius, the king of Armenia, to Christianity. Enraged by the monarch's conversion, and fearing a Roman backlash, king Polymius's brother, prince Astyages, ordered Bartholomew's torture and execution, which Bartholomew courageously endured. However, there are no records of any Armenian King of the Arsacid dynasty of Armenia with the name Polymius. Current scholarship indicates that Bartholomew more likely died in Kalyan in India, where there was an official named Polymius.

Armenia became the first country to establish Christianity as its state religion when, in an event traditionally dated to 301, Gregory the Illuminator convinced Tiridates III, the king of Armenia, to convert to Christianity.

As a consequence of Diocletian's victory over the Sassanids, all of Armenia was once again a vassal state of Rome by 299: Rome secured in this way a wide zone of cultural influence east of Anatolia, which led to a wide diffusion of Syriac Christianity from a center at Nisibis in the first decades of the 4th century, and to the eventual full Christianization of Armenia.

Before this, the dominant religion in Armenia was Zoroastrianism (promoted by the Parthian/Sassanid Empire) and to a smaller degree local Paganism. St Gregory and his son Aristaces were successful in the full Christianization of all Armenians in the first half of the 4th century, mainly after Roman emperor Constantine legalised Christianity in the Roman Empire in 313.

It is a well recognized historical fact that the Armenians were the first nation in the world to formally adhere to Christianity. This conversion was followed in the 4th and 5th centuries by a process of institutionalization and Armenization of Christianity in Armenia. Indeed, Gregory the Illuminator became the organizer of the Armenian Church hierarchy. From that time, the heads of the Armenian Church have been called Catholicos and still hold the same title.

St. Gregory chose as the site of the "Catholicosate" the capital city of Vagharshapat (actual Ejmiatsinin) in Armenia and built there the Etchmiadzin Cathedral as a vaulted basilica in 301-303 (Vahan Mamikonian, Roman governor of Armenia, in 480 ordered the dilapidated basilica to be replaced with a new cruciform church, still standing in the modern Armenia).

The continuous upheavals, which characterized the political scenes of Armenia in the next centuries, made the political power move to safer places often related to the Eastern Roman Empire. The Church center moved as well to different locations together with the political authority, ending in Byzantine Cilicia in the 13th century.