Alexius III Angelus, Byzantine emperor, was the second son of Andronicus Angelus, nephew of Alexius I.

In 1195, while his brother Isaac II was away hunting in Thrace, he was proclaimed emperor by the troops; he captured Isaac at Stagira in Macedonia, put out his eyes, and kept him henceforth a close prisoner, though he had been redeemed by him from captivity at Antioch and loaded with honours.

To compensate for this crime and to confirm his position as emperor, he had to scatter money so lavishly as to empty his treasury, and to allow such licence to the officers of the army as to leave the Empire practically defenceless. He consummated the financial ruin of the state. The able and forceful empress Euphrosyne tried in vain to sustain his credit and his court; Vatatzes, the favourite instrument of her attempts at reform, was assassinated by the emperor's orders.

Eastward the Empire was overrun by the Seljuk Turks; from the north Bulgarians and Vlachs descended unchecked to ravage the plains of Macedonia and Thrace; while Alexius squandered the public treasure on his palaces and gardens. Soon he was threatened by a new and yet more formidable danger. In 1202 the Western princes of the Fourth Crusade assembled at Venice, bent on a new crusade. Alexius, son of the deposed Isaac, escaped from Constantinople and appealed to the crusaders, promising as a crowning bribe to heal the schism of East and West if they would help him to depose his uncle.

The crusaders, whose objective had been Egypt, were persuaded to set their course for Constantinople, before which they appeared in June 1203, proclaiming Alexius as emperor Alexius IV and summoning the capital to depose his uncle. Alexius III, sunk in debauchery, took no efficient measures to resist. His son-in-law, Lascaris, who was the only one to do anything, was defeated at Scutari, and the siege of Constantinople began. On the July 17 the crusaders, the aged doge Enrico Dandolo at their head, scaled the walls and took the city by storm. During the fighting and carnage that followed Alexius hid in the palace, and finally, with one of his daughters, Irene, and such treasures as he could collect, got into a boat and escaped to Develton in Thrace, leaving his wife, his other daughters and his Empire to the victors. Isaac, drawn from his prison and robed once more in the imperial purple, received his son in state.

Shortly afterwards Alexius made an effort in conjunction with Murtzuphlos (Alexius V) to recover the throne. The attempt was unsuccessful and, after wandering about Greece, he surrendered with Euphrosyne, who had meanwhile joined him, to Boniface of Montferrat, then master of a great part of the Balkan peninsula (the so-called Kingdom of Thessalonica). Leaving his protection he sought shelter with Michael, despot of Epirus, and then repaired to Asia Minor, where his son-in-law Lascaris was holding his own against the Latins.

Alexius, joined by Kay Khusrau I , the sultan of Rüm (also called the sultan of Iconium or Konya), now demanded the crown of Lascaris, and on his refusal marched against him. Lascaris, however, defeated and took him prisoner. Alexius was relegated to a monastery at Nicaea, where he died on some date unknown.