Ioannes, (Latin: Iohannes Augustus) known in English as Joannes or even John, was a Roman usurper (423-425) against Valentinian III.
On the death of the Emperor Honorius (15 August 423), Theodosius II, the remaining ruler of the House of Theodosius, hesitated in announcing his uncle's death. In the interregnum, Honorius's patrician at the time of his death, Castinus, elevated Joannes as emperor.
Joannes was a primicerius notariorum or senior civil servant at the time of his elevation. Procopius praised him as "both gentle and well-endowed with sagacity and thoroughly capable of valorous deeds." Unlike the Theodosian emperors, he tolerated all Christian sects. Joannes was proclaimed at Rome and praetorian games were provided at the expense of a member of the gens Anicia. Johannes then moved his base of operations to Ravenna, knowing full well that the Eastern Empire would strike from that direction. There is a mention of an expedition against Africa, but its fate, presumed unsuccessful, is unrecorded.
Joannes had hoped that he could come to an agreement with the Eastern Emperor, but when Theodosius II elevated the young Valentinian III, first to Caesar, then to co-emperor as an Augustus (undoubtedly influenced by Valentinian's mother Galla Placidia), he knew he could only expect war. The army of the Eastern Empire left Thessalonica for Italy, and soon camped in Aquileia. Although the primary sources state that Ravenna fell to their assault, some believe that Ardaburius convinced the garrison of Ravenna to betray the city. The fallen emperor was brought to Aquileia where first his hand was cut off, then he was paraded on a donkey in the Hippodrome to the insults of the populace. After further insults and injuries, Joannes was finally decapitated in June or July 425.