Alexius IV Angelus (c. 1182-1204) was Byzantine Emperor from July 1203 to January 1204. He was the son of emperor Isaac II and nephew of Alexius III.

The young Alexius was imprisoned in 1195 when Alexius III overthrew Isaac II in a coup. He escaped during the disorders of 1201 and fled to his brother-in-law Philip of Swabia, who was married to his sister Irene. While there he met with Boniface of Montferrat, Philip's cousin, who had been chosen to lead the Fourth Crusade, but had temporarily left the Crusade during the siege of Zara to visit Philip. Boniface and Alexius discussed diverting the Crusade to Constantinople so that Alexius and his father could be restored to the throne; in return, Alexius would give them Byzantine soldiers to help fight the Crusade, as well as money to pay off the Crusaders' debt to the Venetians. The Venetians were in favour of this plan when they learned of it, and in 1202 the fleet arrived at Constantinople. Alexius was paraded outside the walls, but the citizens were apathetic, as Alexius III, though a usurper and illegitimate in the eyes of the westerners, was an acceptable emperor for the Byzantine citizens.

In 1203 the Crusaders launched an assault on the city, and Alexius III immediately fled. The next morning the Crusaders were surprised to find that the citizens had released Isaac from prisoner and re-crowned him as emperor, despite the fact that he had been blinded to make him ineligible to rule. The Crusaders could not accept this, and forced Isaac to proclaim Alexius IV co-emperor. Despite Alexius' grand promises, Isaac, the more experienced and practical of the two, knew that the Crusaders' debt could never be repaid from the imperial treasury. Alexius however had apparently not grasped how far the empire's financial resources had fallen during the previous 50 years.

Alexius did manage to raise half the sum promised, by taking from the church and by confiscating property of his enemies. He then attempted to defeat his uncle Alexius III, who remained in control of Thrace. The sack of some Thracian towns helped somewhat, but meanwhile hostility between the restive crusaders and the inhabitants of Constantinople was growing.

In January 1204 Alexius IV was deposed by the leader of the anti-western party, Alexius Murtzouphlos. Isaac II soon died, possibly of old age but possibly from poison, and Alexius IV was strangled. Murtzouphlos was proclaimed emperor as Alexius V.