Theodosius II (Ancient Greek: Θεοδόσιος Theodosios; 10 April 401 – 28 July 450) was Roman emperor from 402 to 450.

He was proclaimed Augustus as an infant and ruled as the Eastern Empire's sole emperor after the death of his father, Arcadius, in 408.

[4] On 10 January 402, at the age of 9 months, he was proclaimed co-augustus by his father,[5] thus becoming the youngest to bear the imperial title up to that point.

On 23 October 425, Valentinian III was installed as emperor of the West with the assistance of the magister officiorum Helion, with his mother taking an influential role.

However, a separation ultimately occurred between the imperial couple between 441 and 444, with Eudocia's establishment in Jerusalem where she favored monastic Monophysitism.

[10] The sixth-century historian John Malalas of Antioch explains Eudocia's departure in a legend involving a certain Phrygian apple.

But the contemporary East Roman diplomat and historian Priscus and a sixth-century chronicler Marcellinus Comes relate a different story.

The situation between the Romans and the Sassanids deteriorated in 420 due to the Persian persecution of Christians, and the Eastern empire declared war against the Sassanids (421–422); the war ended in an indecisive stalemate, when the Romans were forced to accept peace as the Huns menaced Constantinople.

Early in Theodosius II's reign Romans used internal Hun discord to overcome Uldin's invasion of the Balkans.

Anatolius negotiated a peace agreement; the Huns withdrew in exchange for humiliating concessions, including an annual tribute of 2,100 Roman pounds (c. 687 kg) of gold.

[13] In 447 the Huns went through the Balkans, destroying among others the city of Serdica (Sofia) and reaching Athyra (Büyükçekmece) on the outskirts of Constantinople.

Constantinopolitan abbot Eutyches reignited the theological dispute almost twenty years later by asserting the Monophysite view that Christ's divine and human nature were one.

Eutyches was condemned by Archbishop Flavian of Constantinople but supported by the powerful Dioscurus of Alexandria, Cyril's successor.

On 25 November, his sister Pulcheria married the newly elected emperor Marcian, a domesticus under the influential general Aspar.

[18] According to Theodorus Lector, Theodosius was so unmindful of his surroundings that he accidentally signed his sister's note selling his wife, Eudocia, into slavery.

[20] Historian Christopher Kelly argues that "the reign of Theodosius II should not be too quickly dismissed, simplified or partitioned".

[22] Saint Right-Believing Theodosius II the Younger is commemorated in Eastern Orthodox Church on 29 July.