The Roman emperor was the ruler and monarchial head of state of the Roman Empire during the imperial period (starting with the granting of the title augustus to Octavian in 27 BC). The emperors used a variety of different titles throughout history. Often when a given Roman is described as becoming "emperor" in English, it reflects his taking of the title augustus (and later basileus). Another title often used was caesar, used for heirs-apparent, and imperator, originally a military honorific. Early emperors also used the title princeps civitatis ("first citizen"). Emperors frequently amassed republican titles, notably princeps senatus, consul, and pontifex maximus.

The legitimacy of an emperor's rule depended on his control of the army and recognition by the Senate; an emperor would normally be proclaimed by his troops, or invested with imperial titles by the Senate, or both. The first emperors reigned alone; later emperors would sometimes rule with co-emperors and divide administration of the empire between them.

The Romans considered the office of emperor to be distinct from that of a king. The first emperor, Augustus, resolutely refused recognition as a monarch. For the first three hundred years of Roman emperors, from Augustus until Diocletian, efforts were made to portray the emperors as leaders of the republic, fearing any association with the kings of Rome prior to the Roman Republic.

From Diocletian, whose tetrarchic reforms also divided the position into one emperor in the West and one in the East, until the end of the Empire, emperors ruled in an openly monarchic style and did not preserve the nominal principle of a republic, but the contrast with "kings" was maintained: although the imperial succession was generally hereditary, it was only hereditary if there was a suitable candidate acceptable to the army and the bureaucracy, so the principle of automatic inheritance was not adopted. Elements of the republican institutional framework (senate, consuls, and magistrates) were preserved even after the end of the Western Empire.

Constantine the Great moved the capital (Caput Mundi) from Rome to Constantinople, formerly known as Byzantium, in 330 AD and converted to Christianity. After this, the emperor came to be seen as God's chosen ruler, as well as a special protector and leader of the Christian Church on Earth, although in practice an emperor's authority on Church matters was subject to challenge. The Western Roman Empire collapsed in the late 5th century after multiple invasions of imperial territory by Germanic barbarian tribes. Romulus Augustulus is often considered to have been the last emperor of the West, until his forced abdication in 476, although Julius Nepos maintained a claim recognized by the Eastern Empire to the title until his death in 480. Following Nepos's death, the Eastern emperor Zeno abolished the division of the position and proclaimed himself as the sole emperor of a reunited Roman Empire. The subsequent Eastern emperors ruling from Constantinople continued to style themselves "Emperor of the Romans" (later βασιλεύς Ῥωμαίων in Greek), but are often referred to in modern scholarship as Byzantine emperors.

The papacy and Germanic kingdoms of the West acknowledged the Eastern Emperors until the accession of Empress Irene in 797 and the Papacy then created a rival lineage of Roman emperors in western Europe, the Holy Roman Emperors, which ruled the Holy Roman Empire for most of the period between 800 and 1806. These emperors were never recognized as Roman emperors by the court in Constantinople and their coronations resulted in the medieval problem of two emperors. Most western historians treat Constantine XI Palaiologos, who died during the Fall of Constantinople to the Ottoman Empire in 1453 as the last meaningful claimant to the title Roman emperor. From 1453, one of the titles used by the Ottoman Sultans was "Caesar of Rome" part of their titles until the Ottoman Empire ended in 1922. A Byzantine group of claimant Roman emperors existed in the Empire of Trebizond until its conquest by the Ottomans in 1461, though they had used a modified title since 1282.