Honorius (/hoʊˈnɔːriəs/; 9 September 384 – 15 August 423) was Roman emperor from 393 to 423. He was the younger son of emperor Theodosius I and his first wife Aelia Flaccilla. After the death of Theodosius in 395, Honorius, under the regency of Stilicho, ruled the western half of the empire while his brother Arcadius ruled the eastern half. His reign over the Western Roman Empire was precarious and chaotic. In 410, Rome was sacked for the first time since the Battle of the Allia almost 800 years prior.

Honorius was born to Emperor Theodosius I and Empress Aelia Flaccilla on 9 September 384 in Constantinople. He was the brother of Arcadius and Pulcheria. In 386, his mother died, and in 387, Theodosius married Galla who had taken a temporary refuge in Thessaloniki with her family, including her brother Valentinian II and mother Justina, away from usurper Magnus Maximus. Theodosius and Galla had a daughter, Honorius's half-sister Galla Placidia. Honorius, Arcadius, and Galla Placidia were the only children of Theodosius to survive into adulthood.

After holding the consulate at the age of two in 386, Honorius was declared augustus by his father Theodosius I, and thus co-ruler, on 23 January 393, after the death of Valentinian II and the usurpation of Eugenius. When Theodosius died in January 395, Honorius and Arcadius divided the Empire. Honorius became Western Roman emperor at the age of ten.

During the early part of his reign, Honorius depended on the military leadership of the general Stilicho, who had been appointed by Theodosius and was of mixed Vandal and Roman ancestry. To strengthen his bonds with the young emperor and to make his grandchild an imperial heir, Stilicho married his daughter Maria to Honorius. The epithalamion written for the occasion by Stilicho's court poet Claudian survives. Honorius was also influenced by the Popes of Rome: Pope Innocent I and Western bishops may have successfully persuaded Honorius to write to his brother, arguing for convening a synod in Thessalonica.

At first Honorius based his capital in Milan, but when the Visigoths under King Alaric I entered Italy in 401 he moved his capital to the coastal city of Ravenna, which was protected by a ring of marshes and strong fortifications. While the new capital was easier to defend, it was poorly situated to allow Roman forces to protect Central Italy from the increasingly regular threat of barbarian incursions. It is significant that the Emperor's residence remained in Ravenna until the overthrow of Romulus Augustulus in 476. The greater ease with which Ravenna could be defended was probably the reason why Ravenna was chosen not only as the capital of the Ostrogothic Kingdom in Italy, but also for the seat of the Byzantine exarchs as well.

Honorius's reign experienced continued barbarian incursions into Gaul, Italy and Hispania. At the same time, a host of usurpers rose up.

First Gildo, the comes of Africa and magister utriusque militiae per Africam in Northern Africa, led a revolt which lasted for two years (397–398). Stilicho eventually subdued it through the local command of Mascezel, Gildo's brother.

The next crisis was the Visigothic invasion of Italy in 402 under the command of their king, Alaric I. Stilicho was absent in Raetia in the latter months of 401, when Alaric, who was also the Eastern Empire's magister militum in Illyricum, suddenly marched with a large army through the Julian Alps and entered Italy.