Flavius Theodosius was a senior military officer serving in the Western Roman Empire. He held the rank of comes, and as such is usually referred to as Count Theodosius. He is credited with being the founding member of the House of Theodosius.

He married Thermantia, by whom he had a son, also named Theodosius, at Cauca in Spain around 346. All three were Catholics.

He was sent to Britain in 368 to deal with a barbarian invasion of the Roman provinces in that island, which he dealt with successfully. On his return, he succeeded Jovinus as the magister equitum praesentalis at the court of the emperor Valentinian I. In 373 he was made commander of the expedition to suppress the rebellion of Firmus in Mauretania, but following his victory over Firmus, Theodosius was arrested, taken to Carthage, and executed in early 375. The reasons for his execution are not clear.

After a period of retirement on the family estates in Gallicia, his son Theodosius was elevated to Emperor January 19, 379.