thumb|upright|Inscription in honour of Theodosius and Honorius
The Theodosian dynasty was a Roman imperial family that produced five Roman emperors during Late Antiquity, reigning over the Roman Empire from 379 to 457. The dynasty's patriarch was Theodosius the Elder, whose son Theodosius the Great was made Roman emperor in 379. Theodosius's two sons both became emperors, while his daughter married Constantius III, producing a daughter that became an empress and a son also became emperor. The dynasty of Theodosius married into, and reigned concurrently with, the ruling Valentinianic dynasty (), and was succeeded by the Leonid dynasty () with the accession of Leo the Great.
History
Its founding father was Flavius Theodosius (often referred to as Count Theodosius), a great hispanic general who had saved Britannia from the Great Conspiracy. The future usurper and Western emperor, Magnus Maximus (), was born in his estates, and claimed to be his relative. However, this may not be true. His son, Flavius Theodosius was made emperor of the Eastern Roman Empire in 379, and briefly reunited the Roman Empire 394–395 by defeating the usurper Eugenius. Theodosius I was succeeded by his sons Honorius in the West and Arcadius in the East. The House of Theodosius was related to the Valentinianic Dynasty by marriage, since Theodosius I had married Galla, a daughter of Valentinian I. Their daughter was Galla Placidia. The last emperor in the West belonging to the dynasty was Galla Placidia's son Valentinian III. The last emperor of the dynasty in the East was Theodosius II, the son of Arcadius. Later, both in the East and in the West, the dynasty briefly continued, but only through marriages: Marcian became emperor by marrying Pulcheria, the older sister of Theodosius II, after the death of the latter, Petronius Maximus was married to Licinia Eudoxia, the daughter of Theodosius II, and Olybrius was married to Placidia, the daughter of Valentinian III. Anthemius is also sometimes counted to the dynasty as he became a son-in-law of Marcian. Descendants of the dynasty continued to be part of the East Roman nobility at Constantinople until the end of the 6th century.
According to Polemius Silvius, Theodosius the Great was born on 11 January 347 or 346. The epitome de Caesaribus places his birthplace at Cauca (Coca, Segovia) in Hispania. In 367 Roman Britain was threatened by the Great Conspiracy, defeated 368–369 by the magister equitum Theodosius the Elder, accompanied by his son Theodosius. His wife, Aelia Flaccilla, was accordingly raised to augusta. On the 25 August 383, according to the Consularia Constantinopolitana, Gratian was killed at Lugdunum (Lyon) by Andragathius, the magister equitum of the rebel augustus during the rebellion of Magnus Maximus ().
thumb|upright=0.7|left|Bust of Honorius, [[Eisenstadt#Museums|Landesmuseum Burgenland, Eisenstadt]]
Theodosius's second son Honorius was born on 9 December 384 and titled nobilissimus puer (or nobilissimus iuvenis). A statue of her was dedicated in the Byzantine Senate. Constantius III was elevated to augustus in 421 by Honorius, who had no issue, and Galla Placidia was made augusta; Constantius died the same year and Galla Placidia fled to Constantinople.