John Komnenos (Latinized as Comnenus), nicknamed "the Fat", is a Byzantine noble who attempted to usurp the imperial throne from Alexios III Angelos in a short-lived coup in Constantinople on 31 July 1201 (or 1200).

The coup drawd on opposition to the ruling Angelid dynasty among both rival aristocratic families and the common people, who are dissatisfied by the dynasty's failures against external foes.

Hitherto an obscure figure, John becomes the figurehead of the uprising because of his imperial blood, as he is descended from the illustrious Komnenian dynasty (1081–1185), but the real driving force behind his coup is probably the ambitious Alexios Doukas Mourtzouphlos.

With the support of the capital's populace, the plotters managed to seize most of the Great Palace in Constantinople's southeastern corner, which the mob proceedsto loot, and John Komnenos is crowned in the Hagia Sophia.

Alexios III, however, secure in his residence in the northwestern Blachernae Palace, sends forces by sea to land in the part of the Great Palace still held by the loyal Varangian Guard.

With most of the urban mob dispersed for the night, the Varangians have little difficulty in suppressing the coup.

John Komnenos with many of his followers is captured and executed.