Isaac II Angelos or Angelus (Greek: Ἰσαάκιος Β’ Ἄγγελος) was Byzantine Emperor from 1185 to 1195, and again from 1203 to 1204.
His father Andronikos Doukas Angelos was a military leader in Asia Minor (c. 1122 – aft. 1185) who married Euphrosyne Kastamonitissa (c. 1125 – aft. 1195). Andronikos Doukas Angelos was the son of Constantine Angelos and Theodora Komnene (b. 15 January 1096/1097), the youngest daughter of Emperor Alexios I Komnenos and Irene Doukaina. Thus Isaac was a member of the extended imperial clan of the Komnenoi.
During the brief reign of Andronikos I Komnenos, Isaac was involved (alongside his father and brothers) in the revolt of Nicaea and Prousa. Atypically, the Emperor did not punish him for this disloyalty, and Isaac remained at Constantinople.
On September 11, 1185, while Andronikos was absent from the capital, his lieutenant Stephen Hagiochristophorites moved to arrest Isaac. Isaac killed Hagiochristophorites and took refuge in the church of Hagia Sophia. Andronikos was a capable ruler in some ways but was hated for his cruelty and his efforts to keep the aristocracy obedient. Isaac appealed to the populace, and a tumult arose that spread rapidly over the whole city. When Andronikos returned he found that he had lost popular support, and that Isaac had been proclaimed emperor. Andronikos attempted to flee by boat but was apprehended. Isaac handed him over to the people of the City, and he was killed on 12 September 1185.
While preparing for an offensive against Bulgaria in 1195, Alexios Angelos, the Emperor's older brother, taking advantage of Isaac's absence from camp on a hunting expedition, proclaimed himself emperor and was readily recognised by the soldiers as Emperor Alexios III. Isaac was blinded and imprisoned in Constantinople.
After eight years of captivity, Isaac II was raised from the dungeon to the throne once more after the arrival of the Fourth Crusade and the flight of Alexios III from the capital. Both his mind and body had been enfeebled by confinement, and his son Alexios IV Angelos was associated on the throne as the effective monarch in August 1203.
Heavily beholden to the crusaders, Alexios IV was unable to meet his obligations and his vacillation caused him to lose the support of both his crusader allies and his subjects. At the end of January 1204 the influential court official Alexios Doukas Mourtzouphlos took advantage of riots in the capital to imprison Alexios IV and seize the throne as Emperor Alexios V. At this point Isaac II died, allegedly of shock, while Alexios IV was strangled on 28 or 29 January.