The Battle of the Frigidus, also called the Battle of the Frigid River, is fought between September 5–6, 394, between the army of the Eastern Emperor Theodosius I and the army of Western Roman ruler Eugenius.The defeat of Eugenius and his commander, the Frankish magister militum Arbogast, puts the whole empire back in the hands of a single emperor for the last time in Roman history.

Most significantly, the battle is the last attempt to contest the Christianization of the empire; its outcome decided the outcome of Christianity in the western Empire.