Isaac II Angelus, Byzantine emperor 1185-1195, and again 1203-1204, was the successor of Andronicus I.

He inaugurated his reign by a decisive victory over the Normans in Sicily, but elsewhere his policy was less successful. He failed in an attempt to recover Cyprus from the rebellious noble Isaac Comnenos thanks to Norman interference and by the oppressiveness of his taxes drove the Bulgarians and Vlachs to revolt (1186). In 1187 Alexis Branas , the general sent against the rebels, treacherously turned his arms against his master, and attempted to seize Constantinople, but was defeated and slain. The emperor's attention was next demanded in the east, where several claimants to the throne successively rose and fell. In 1189 Frederick I, Holy Roman Emperor sought and obtained leave to lead his troops on the Third Crusade through the Byzantine Empire; but he had no sooner crossed the border than Isaac, who had meanwhile sought an alliance with Saladin, threw every impediment in his way, and was only compelled by force of arms to fulfil his engagements.

The next five years were disturbed by fresh rebellions of the Vlachs, against whom Isaac led several expeditions in person. During one of these, in 1195, Alexius, the emperor's brother, taking advantage of the latter's absence from camp on a hunting expedition, proclaimed himself emperor, and was readily recognised by the soldiers. Isaac was blinded and imprisoned in Constantinople. After eight years he was raised for six months from his dungeon to his throne once more after the arrival of the knights of the Fourth Crusade. But both mind and body had been enfeebled by captivity, and his son Alexius IV was the actual monarch. Isaac died in 1204, shortly after the usurpation of his general, Mourzouphles. He was one of the weakest and most vicious princes that occupied the Byzantine throne. Surrounded by a crowd of slaves, mistresses and flatterers, he permitted his empire to be administered by unworthy favourites, while he squandered the money wrung from his provinces on costly buildings and expensive gifts to the churches of his metropolis.