Alexius is the latinized form of the given name Alexios (Αλέξιος, polytonic Ἀλέξιος, "defender", cf. Alexander), especially common in the later Byzantine Empire. Variants include Alexis and the Russian derivative Aleksey. The female form is Alexia (Αλεξία).

It may refer to:

Rulers

• Alexius I Comnenus (1048-1118), Byzantine emperor
• Alexius II Comnenus (1167-1183), Byzantine emperor
• Alexius III, Byzantine emperor
• Alexius IV, Byzantine emperor
• Alexius V, Byzantine emperor
• Alexius I of Trebizond, Emperor of Trebizond
• Alexius II of Trebizond, Emperor of Trebizond
• Alexius III of Trebizond, Emperor of Trebizond
• Alexius IV of Trebizond, Emperor of Trebizond
• Alexius Mikhailovich (1629-1676), Tsar of Russia
• Alexius Petrovich (1690-1718), Russian tsarevich

Religious figures

• Alexius, Metropolitan of Moscow, (1354-1378)
• Patriarch Alexius I of Constantinople (1025-1043)
• Patriarch Alexius II (1990-present), Patriarch of Moscow and all Russia
• Alexius of Nicaea, metropolitan bishop
• Saint Alexius of Rome, fifth-century eastern saint
• Alexius, a monk and saint of Kiev - see Abraham and Onesimus of Kiev

Other people

• Alexios Apokaukos, Byzantine statesman
• Alexios Branas
• Alexios Philanthropenos, Byzantine general
• Alexios Strategopoulos, Byzantine general