Alexios IV Angelos or Alexius IV Angelus (Greek: Αλέξιος Δ' Άγγελος) was Byzantine Emperor from August 1203 to January 1204. He was the son of Emperor Isaac II Angelus and his first wife, an unknown Palaiologina, who became a nun with the name Irene. His paternal uncle was Emperor Alexius III Angelus.
The young Alexios was imprisoned in 1195 when Alexios III overthrew Isaac II in a coup. In 1201, two Pisan merchants were employed to smuggle Alexius out of Constantinople to the Holy Roman Empire, where he took refuge with his brother-in-law Philip of Swabia, King of Germany.
According to the contemporary account of Robert of Clari it was while Alexius was at Swabia's court that he met with Marquis 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 Alexios allegedly discussed diverting the Crusade to Constantinople so that Alexios could be restored to his father's throne. Montferrat returned to the Crusade while it wintered at Zara and he was shortly followed by Prince Alexios's envoys who offered to the Crusaders 10,000 Byzantine soldiers to help fight in the Crusade, maintain 500 knights in the Holy Land, the service of the Byzantine navy (20 ships) in transporting the Crusader army to Egypt, as well as money to pay off the Crusaders' debt to the Republic of Venice with 200,000 silver marks. Additionally, he promised to bring the Greek Orthodox Church under the authority of the pope. The Venetians and most of the Leaders were in favour of the plan however some were not and there were defections, including Simon of Montfort. In 1202 the fleet arrived at Constantinople. Alexios was paraded outside the walls, but the citizens were apathetic, as Alexios III, though a usurper and illegitimate in the eyes of the westerners, was an acceptable emperor for the Byzantine citizens.
On 18 July 1203 the Crusaders launched an assault on the city, and Alexios III immediately fled into Thrace. The next morning the Crusaders were surprised to find that the citizens had released Isaac II from prison and proclaimed him emperor, despite the fact that he had been blinded to make him ineligible to rule. The Crusaders could not accept this, and forced Isaac II to proclaim his son Alexios IV co-emperor on 1 August.
Despite Alexios' grand promises, Isaac, the more experienced and practical of the two, knew that the Crusaders' debt could never be repaid from the imperial treasury. Alexios, however, had apparently not grasped how far the empire's financial resources had fallen during the previous fifty years. Alexios did manage to raise half the sum promised (100,000 silver marks), by appropriating treasures from the church and by confiscating the property of his enemies. He then attempted to defeat his uncle Alexios III, who remained in control of Thrace. The sack of some Thracian towns helped Alexios' situation a little, but meanwhile hostility between the restive Crusaders and the inhabitants of Constantinople was growing.
In December 1203 violence exploded between the citizens of Constantinople and the Crusaders. At the end of January 1204, the populace of Constantinople rebelled and tried to proclaim a rival emperor in Hagia Sophia. Alexios IV attempted to reach a reconciliation with the Crusaders, entrusting the anti-western courtier Alexios Doukas Murzuphlus with a mission to gain Crusader support. However, Alexios Doukas imprisoned both Alexios IV and his father on the night of 27–28 January 1204. Isaac II died soon afterwards, possibly of old age or from poison, and Alexios IV was strangled on 8 February. Alexios Doukas was proclaimed emperor as Alexios V. During Alexios IV's brief reign, the empire lost its territories along the Black Sea coast to the Empire of Trebizond.