Ioannes, known in English as Joannes, was a Roman usurper (423–425) against Valentinian III.
On the death of the Emperor Honorius (August 15, 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. His control over Gaul was insecure: his pretorian prefect of that region was slain at Arles in an uprising of the soldiery there. And Comes Bonifacius, in control of the African provinces, held back the grain fleet from Rome.
"The events of Johannes' reign are as shadowy as its origins," writes John Matthews, who then provides a list of the ruler's known actions in a paragraph. 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. In Gaul, he appears to have caused offense by submitting clerics to secular courts. And that is all.
Joannes had hoped that he could come to an agreement with the emperor Theodosius, 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. Late in 424, he gave one of his younger but promising, followers Aëtius an important mission. Aëtius, at the time a Governor of the Palace, was sent to the Huns, with whom he had lived as a hostage, to seek military help.
While Aëtius was away on his mission, the army of the Eastern Empire left Thessalonica for Italy, and soon made their base in Aquileia. Although the primary sources state that Ravenna fell to their assault -- John of Antioch states that a shepherd led the army of Aspar safely through the marshes that protected the city -- Stewart Oost believes that Aspar's father, Ardaburius, who had been captured by Joannes' soldiers, 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, then after further insults and injuries he was decapitated in June or July 425.
Three days after Joannes's death, Aëtius returned at the head of a substantial Hunnic army. After some skirmishing, Placidia and Aëtius came to an agreement that established the political landscape of the Western Roman Empire for the next thirty years. The Huns were paid off and sent home, while Aetius received the position of magister militum (commander-in-chief of the Roman army).