The Battle of Mediolanum was a battle between the Alemannic Germans and the Roman legions under the command of Emperor Gallienus near Mediolanum (Milan) in 259 AD.

The situation in the Western half of the Empire was really difficult. The Danubian border resisted continuous barbarian attacks. Gallienus had to march with military reinforcements from Gaul towards Dacia and Moesia to fight the Barbarians. The situation was so severe that in 259, the legions of Pannonia and Moesia rebelled and chose to make Ingenuus the emperor. Gallienus reunited the Rhine, left Legio II Parthica to defend it and went off to do battle.

The Alamanni crossed the Alpine steps and attacked the fertile plain of the Po river. The sacking of the area instilled terror in Rome, as it was still not a walled city. The Senate of Rome hastily prepared a crowd of plebs for combat in an attempt to ensure that its shrinking army was capable of protecting the city. Gallienus had just defeated the pretender Ingenuus when the news arrived of the invasion by the Alamanni. He marched off with the legions I Adiutrix, II Italica and II Parthica to intercept the barbarians in Italy. By then, according to the Byzantine historian Joannes Zonaras, the Alamanni had retreated before the unexpected resistance of the citizens of Rome and its Senate. When Gallienus arrived in the valley of the Po, he found the Alamanni in the vicinity of Mediolanum. The victory was total: according to Zonaras 300,000 Alamanni fell that day and the emperor received the title Germanicus Maximus.

The Alamanni's success in reaching into the Roman Empire once more revealed the weakness of the centuries-old tradition of posting Rome's legions near the borders without providing for defence within the empire. The battle of Mediolanum demonstrated to the Romans the value of swift, flexible military units. Afterward, Gallienus enacted a major reform by introducing a highly mobile field army composed mainly of cavalry (vexillationes). The main units were under the control of his General, Aureolus, and headquartered in Mediolanum, with the mission to protect Italy. Finally, the invasion by the Alamanni demonstrated the vulnerability of Italy and especially Rome. This later caused Emperor Aurelian to have a strong wall built to defend the capital of the Empire.