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.