Develtos (Bulgarian: Деултум, Greek: Δεβελτός, Δηβελτός, Δεουελτός, Δεούελτος, Διβηλτóς) or Deultum 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.

Develton (Thracian: Debelton, "two-swamp area") was founded as an emporium of Apollonia Pontica in the 7th century BC. From the 6th century to the 4th century BC, the settlement served as an important place of trade between Thracians and Greeks.

Develton was annexed to the Roman Empire in 46 AD and became part of the province of Thrace. The construction of a colonia for veterans of the VIII Augusta legion at Develton was likely planned prior to 69 AD, but was delayed due to the eruption of civil war of 69 AD. The veterans may have been settled at Develton due to its proximity to the neighbouring region of Moesia, where VIII Augusta was formerly based. The location was also chosen for a veterans colony as the veterans had the appropriate training necessary for the drainage of local marshland, therefore allowing the area to be developed and exploited. The colonia was thus built during the reign of Emperor Vespasian, and was named Colonia Flavia Pacis Deultensium, or Colonia Flavia Pancensis Deultum. The inclusion of "peace" (Pacis) in the title of the colony probably referred to the conclusion of the civil war. The city had an extensive territory, as testified by inscriptions in Panchevo and Sladki Kladenci near Burgas.

Pliny the Elder makes reference to the city in his Naturalis Historia. In 82 AD, the population of Deultum petitioned Titus Avidius Quietus to become a patron of the city. Commemorative bronze coins were minted by Emperor Trajan to celebrate the 30th anniversary of the foundation of Deultum. Between 130 and 150 AD, the city suffered serious damage from barbarian attacks.

By the end of the 2nd century and the beginning of the 3rd century, Deultum had an area of approximately 62 acres (0.25 sq km) and there were temples dedicated to Asclepius and Cybele. A mint was active at Deultum from the reign of Caracalla to that of Emperor Philip the Arab. The city is mentioned in the Antonine Itinerary, composed in the early 3rd century. 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.

Deultum was sacked by Goths in the second half of the 3rd century and was rebuilt shortly after. The city later became part of the province of Haemimontus, and Emperor Diocletian travelled through Deultum in 294 whilst en route from Sirmium to Nicomedia. Legions I Flavia Pacis, II Flavia Pacatiana, and III Flavia Pacis may have been levied at Deultum and its environs by Diocletian or Emperor Constantius II.

At the Battle of Deultum in the summer of 377 during the Gothic War of 376–382 an Eastern Roman army was defeated by a Gothic raiding party outside Deultum, and the city was sacked. 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. However, the walls were destroyed by Slavs and Avars at the end of the 6th century.

Develtos lay on the border with Bulgaria after the Treaty of 716 between Emperor Theodosius III and Khan Tervel, and became a significant defensive post in the war with the Bulgarians. As a consequence of the treaty, the city was situated at the edge of a depopulated no man's land known as Zagoria ("beyond the [Haemus] mountains") to the Bulgarians. The office of kommerkia of Develtos is first attested in the 8th century. Zagoria was retaken by Emperor Constantine V in 756, 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.