Marcus Aurelius Severus Alexander (1 October 208 – 21/22 March 235), also known as Alexander Severus, was a Roman emperor, who reigned from 222 until 235. He was the last emperor from the Severan dynasty. He succeeded his slain cousin Elagabalus in 222. 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.

Alexander was the heir to his cousin, the 18-year-old Emperor Elagabalus. The latter had been murdered along with his mother Julia Soaemias by his own guards, who, as a mark of contempt, had their remains cast into the Tiber river.  Alexander and his cousin were both grandsons of Julia Maesa, who was the sister of empress Julia Domna and had arranged for Elagabalus's acclamation as emperor by the Third Gallic Legion.

Alexander's 13-year reign was the longest reign of a sole emperor since Antoninus Pius.  He was also the youngest sole legal Roman emperor during the existence of the united empire. Alexander's peacetime reign was prosperous. However, Rome was militarily confronted with the rising Sassanid Empire and growing incursions from the tribes of Germania. He managed to check the threat of the Sassanids. But when campaigning against Germanic tribes, Alexander attempted to bring peace by engaging in diplomacy and bribery. This alienated many in the Roman army, leading to a conspiracy that resulted in the assassination of Alexander, his mother Julia Avita Mamaea, and his advisors. After their deaths, the accession of Maximinus Thrax followed. Alexander's death marked the epoch event for the Crisis of the Third Century.