Roman emperor
The Western Roman Empire collapsed in the late 5th century. Romulus Augustulus is often considered to be the last emperor of the West after 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' death, the Eastern Emperor Zeno abolished the division of the position and proclaimed himself as the sole Emperor of a reunited Roman Empire. The Eastern imperial lineage continued to rule from Constantinople ("New Rome"); they continued to style themselves as Emperor of the Romans (later βασιλεύς Ῥωμαίων in Greek), but are often referred to in modern scholarship as Byzantine emperors. Constantine XI Palaiologos was the last Roman emperor in Constantinople, dying in the Fall of Constantinople to the Ottomans in 1453.
Western Roman Empire
By convention, the Western Roman Empire is deemed to have ended on 4 September 476, when Odoacer deposed Romulus Augustus, but the historical record calls this determination into question. Indeed, the deposition of Romulus Augustus received very little attention in contemporary times. Romulus was a usurper in the eyes of the Eastern Roman Empire and the remaining territories of Western Roman control outside of Italy, with the previous emperor Julius Nepos still being alive and claiming to rule the Western Empire in Dalmatia. Furthermore, the Western court had lacked true power and had been subject to Germanic aristocrats for decades, with most of its legal territory being under control of various barbarian kingdoms. With Odoacer recognising Julius Nepos, and later the Eastern Emperor Zeno, as his sovereign, nominal Roman control continued in Italy. Syagrius, who had managed to preserve Roman sovereignty in an exclave in northern Gaul (a realm today known as the Domain of Soissons) also recognized Nepos as his sovereign and the legitimate Western Emperor.