Theodosius II (Latin: Flavius Theodosius Iunior Augustus; 10 April 401 – 28 July 450), commonly surnamed Theodosius the Younger, or Theodosius the Calligrapher, was Eastern Roman Emperor from 408 to 450. He is mostly known for promulgating the Theodosian law code, and for the construction of the Theodosian Walls of Constantinople. He also presided over the outbreak of two great christological controversies, Nestorianism and Eutychianism.

Theodosius was born in 401 as the only son of Emperor Arcadius and his Frankish-born wife Aelia Eudoxia. Already in January AD 402 he was proclaimed co-Augustus by his father, thus becoming the youngest person ever to bear this title in Roman history. In 408, his father died and the seven-year-old boy became Emperor of the Eastern half of the Roman Empire.

Government was at first by the Praetorian Prefect Anthemius, under whose supervision the Theodosian land walls of Constantinople were constructed.

In 414, Theodosius' older sister Pulcheria was proclaimed Augusta and assumed the regency. By 416 Theodosius was declared Augustus in his own right and the regency ended, but his sister remained a strong influence on him. In June 421, Theodosius married Aelia Eudocia, a woman of Greek origin. The two had a daughter named Licinia Eudoxia. A separation ultimately occurred between the imperial couple, with Eudocia's establishment in Jerusalem where she favoured monastic Monophysitism and Pulcheria reassuming an influential role with the support of the eunuch Chrysapius.

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