Severus Alexander (Latin: Marcus Aurelius Severus Alexander Augustus; 1 October 208 – 18 or 19 March 235) was Roman Emperor from 222 to 235. Alexander was the last emperor of the Severan dynasty. He succeeded his cousin Elagabalus upon the latter's assassination in 222, and was ultimately assassinated himself, marking the epoch event for the Crisis of the Third Century — nearly fifty years of civil wars, foreign invasion, and collapse of the monetary economy.

Alexander was the heir apparent to his cousin, the eighteen-year-old Emperor who had been murdered along with his mother by his own guards, who, as a mark of contempt, had their remains cast into the Tiber river. He and his cousin were both grandsons of the influential and powerful Julia Maesa, who had arranged for Elagabalus' acclamation as emperor by the famed Third Gallic Legion. It was the rumor of Alexander's death that triggered the assassination of Elagabalus and his mother .

As emperor, Alexander's peace time reign was prosperous. In military conflict against the rising Sassanid Empire, there are mixed accounts, though the Sassanid threat was checked; however, when campaigning against Germanic tribes of Germania, Alexander apparently alienated his legions by engaging in diplomacy and bribery, and they assassinated him.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.