Theodosius I (Latin: Flavius Theodosius Augustus; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman Emperor from 379 to 395. Theodosius was the last emperor to rule over both the eastern and the western halves of the Roman Empire. During his reign, the Goths secured control of Illyricum after the Gothic War, establishing their homeland south of the Danube within the empire's borders. He also issued decrees that effectively made Christianity the official state religion of the Roman Empire.

He is recognized by the Eastern Orthodox Church as Saint Theodosius. He defeated the usurpers Magnus Maximus and Eugenius and fostered the destruction of some prominent pagan temples: the Serapeum in Alexandria, the Temple of Apollo in Delphi, and the Vestal Virgins in Rome. After his death, Theodosius' sons Arcadius and Honorius inherited the East and West halves respectively, and the Roman Empire was never again re-united.

Theodosius was born in Cauca or Italica, Hispania, to senior military officer Theodosius the Elder. He accompanied his father to Britannia to help quell the Great Conspiracy in 368. He was military commander (dux) of Moesia, a Roman province on the lower Danube, in 374. However, shortly thereafter, and at about the same time as the sudden disgrace and execution of his father, Theodosius retired to Spain. The reason for his retirement, and the relationship (if any) between it and his father's death is unclear. It is possible that he was dismissed from his command by the emperor Valentinian I after the loss of two of Theodosius' legions to the Sarmatians in late 374.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.

Pope Theodosius I of Alexandria (died June 5, 567) was the last Patriarch of Alexandria recognised by both Copts and Melchites.

As successor to Timothy III (IV), he was at first recognized by the Emperor Justinian I and the Eastern Orthodox Church. However, because of his Miaphysite theology, he was rejected by the Eastern Orthodox Church of Alexandria and exiled by the Emperor Justinian I in 536. In his place, Paul was elected Patriarch.

As the Copts continued to recognise Timothy, the lineage between the Coptic and Melchite split. This split endures until today.

Theodosius spent the last 28 years of his life imprisoned in Upper Egypt and after his death the Coptic Church elected Peter IV as his successor.

Theodosius is commemorated in the Coptic Synaxarion on the 28th day of Ba'unah (June 5), the day of his death.