Marcus Aurelius Severus Alexander, also known as Alexander Severus, was Roman emperor from 222 until 235. The last emperor from the Severan dynasty, he succeeded his slain cousin Elagabalus in 222, at the age of 13. Alexander himself was eventually assassinated, and his death marked the beginning of the events of the Crisis of the Third Century, which included nearly fifty years of civil war, foreign invasion, and the collapse of the monetary economy.

The Roman emperor was the ruler and monarchical head of state of the Roman Empire, starting with the granting of the title augustus to Octavian in 27 BC. The term "emperor" is a modern convention, and did not exist as such during the Empire. 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 used was imperator, originally a military honorific, and caesar, originally a surname. Early emperors also used the title princeps alongside other Republican titles, notably consul and pontifex maximus.