Develtos (Bulgarian: Деултум, Greek: Δεβελτός, Δηβελτός, Δεουελτός, Δεούελτος, Διβηλτóς) or Deultum [nb 1] was an ancient city and bishopric in Thrace.

It was located at the mouth of the river Sredetska reka on the west coast of Lake Mandrensko, previously part of the Gulf of Burgas, and near the modern village of Debelt.

[12] Following his ascension to the throne, Emperor Philip the Arab travelled through Deultum in 244 en route from Circesium to Rome, and adventus coins were minted to celebrate his presence.

[10] Deultum was later rebuilt on a smaller scale, and, in the second half of the 5th century, new walls were constructed and all unprotected buildings were demolished to ensure hostile forces did not use them as cover.

[10] Develtos lay on the border with Bulgaria after the Treaty of 716 between Emperor Theodosius III and Khan Tervel,[11] and became a significant defensive post in the war with the Bulgarians.

[18] Zagoria was retaken by Emperor Constantine V in 756,[19] and thus, by the early 9th century, the city had become part of a chain of military bases in northern Thrace which helped confine the Bulgarians to the north of the Haemus Mountains.

[24] Emperor Leo V's victory over Khan Omurtag in April 816 near Mesembria led to the negotiation and ratification of a treaty in September, which returned the city to Roman control.

[25] As per the Treaty of 816, the Great Fence (in Bulgarian), a fortified earthwork palisade, was constructed by Bulgaria northwest of Develtos to mark the border between the two states.

[26] Following Bulgarian raids into Thrace in 853, the city was ceded by Empress Theodora to Khan Boris I,[27] by which time Develtos was still partially ruined.

[30] The embassy was tasked with informing the emperor of the pope's condemnation of Patriarch Photios I of Constantinople, and consisted of Donatus, Bishop of Ostia, the deacon Marinus, and the priest Leo.

[31] As per the stipulations of the Treaty of 927, Tsar Peter I relinquished control of the city to Emperor Romanos I Lekapenos, and Develtos became part of the theme of Thrace.

[33] The city was the temporary refuge of Emperor Alexios III Angelos in mid-July 1203 after he had fled Constantinople when faced with the arrival of the Fourth Crusade.

[7] Saint George, Archbishop of Develtos, was amongst those deported after the city's surrender to Krum in 812 and was tortured to death by Khan Omurtag in 815 with roughly 380 other martyrs who refused to renounce their faith.