Eastern Roman emperor from 408 to 450. He was a gentle, scholarly, easily dominated man who allowed his government to be run by a succession of relatives and ministers.

The son of the Eastern emperor Arcadius (reigned 383–408), he was made coemperor in 402 and became sole ruler of the East upon his father's death in 408. At first the able Anthemius, praetorian prefect of the East, was regent for young Theodosius. Anthemius dropped out of sight in 414, when the emperor's sister Pulcheria received the title augusta and assumed the regency. Throughout his reign, control of the government remained out of Theodosius' hands.

At various times during his reign, Theodosius sent armies against the Vandals of Africa, the Persians, and the Huns. His generals defeated Persian (Sāsānian) invaders in 422 and 447, but campaigns against the Vandals, who had occupied most of Roman Africa in 429, ended in failure. Theodosius' policy of appeasing the mighty Hun leader Attila did not prevent massive Hun invasions of the Danubian provinces in 441–443 and 447. His reign was also troubled by a dispute over the heretical doctrines of Nestorius, whom Theodosius appointed patriarch of Constantinople in 428. Nestorius was deposed by a church council in 431.

Theodosius' name is associated with three important projects. The first, erection of an impregnable wall around Constantinople (413), was actually the work of Anthemius. The emperor did, however, have a hand in founding the University of Constantinople in 425 and in supervising compilation of the Theodosian Code (published 438), which codified the laws issued after 312. Theodosius died from injuries suffered during a hunting accident. His daughter Licinia Eudoxia married the Western Roman emperor Valentinian III (reigned 425–455).