Count Theodosius (Latin: Comes Theodosius) was a senior military officer serving in the Western Roman Empire. He is also known as Flavius Theodosius or as Theodosius the Elder, distinguishing him from his son, the Roman emperor Theodosius I. He was granted the title of Count of the Britains (Comes Britanniarum) for his work there putting down the Great Conspiracy. He afterwards fought against the Alemanni and in Mauretania.
There is some evidence that Flavius Theodosius's father was called Honorius. Probably sometime in the late 330s or early 340s he married Thermantia. By her, he had at least two sons, Honorius and Theodosius (afterwards emperor Theodosius I), born at Cauca (modern Coca, Segovia) in Spain. The family were orthodox Christians.
In 368 Flavius Theodosius was first raised to the Roman military rank of comes (akin to a general) and sent to Britannia to recover the lands lost to the Great Conspiracy the year prior. Ammianus Marcellinus records that he put down a rebellion by the Pannonian Valentine. Having done so, he recovered and reärranged its provinces, although many of its people people had been killed or sold into slavery by the invaders. Known to have been with him on this expedition were his younger son Theodosius and, in all likelihood, the future usurper Magnus Maximus.
On his return Count Theodosius succeeded Jovinus as the magister equitum praesentalis at the court of Emperor Valentinian I, in which capacity he prosecuted another successful campaign against the Alemanni in 370.
In 373 Count Theodosius was made commander of the expedition to suppress the rebellion of Firmus in Mauretania, which proved to be yet another victory for the skilled commander. But after this victory he was arrested, taken to Carthage, and put to death in early 376. The reasons for this are not clear, but it is thought to have resulted from a factional power struggle in Italy after the sudden death of emperor Valentinian I in November 375. Shortly before his death Count Theodosius accepted Christian baptism - a common practice at the time, even for lifelong Christians.
After Count Theodosius's death his son Theodosius was sent home to the family estates in Gallaecia (now Galicia in Spain). But two years later, in 378, after the Roman defeat at the Battle of Adrianople, the younger Theodosius was rehabilitated, placed in charge of the Roman armies in the eastern half of the empire, and quickly elevated to the rank of emperor on January 19, 379 following his successes in the field.