Gerontius (died 411) was a general of the Western Roman Empire, who initially supported the usurper Constantine III but later opposed him in favour of another usurper, Maximus of Hispania.
Gerontius, probably a Briton by birth, was one of the supporters of Constantine III, a Roman general who revolted against the Western Roman Emperor Honorius in 407, conquering Britain, Gaul, Germania and Hispania.
In 408 he followed the son of Constantine, the newly appointed caesar Constans II, to Spain. This province was under Constantine's rule, but here some members of the House of Theodosius, the cousins of Honorius, Didymus and Verenianus, had rebelled. Gerontius, who was the actual commander-in-chief of the troops, fought the rebels in two battles. In the first he was defeated, but in the second he won an important victory in Lusitania, where he had recalled some troops from Gaul, and captured the rebel chiefs.
While Constantine returned to Gaul with the prisoners, Gerontius remained in Spain, in command of the local troops.