The settlement was Thracian until the Dorians from Megara colonized it at the beginning of 6th c. BC. It was one of the most important strongholds of the Byzantine Empire from the 5th century AD onwards, and was fought over by Byzantines and Bulgarians captured and incorporated in the lands of the First Bulgarian Kingdom by Khan Krum after a two-week siege only to be ceded back to Byzantium by Knyaz Boris I in 864 and reconquered by his son Tsar Simeon the Great. During the time of the Second Bulgarian Kingdom it was also contested by Bulgarian and Byzantine forces and enjoyed particular prosperity under Bulgarian tsar Ivan Alexander (1331–1371) until it was conquered by Crusaders led by Amadeus VI, Count of Savoy in 1366. The Bulgarian version of the name, Nesebar or Mesebar, has been attested since the 11th century.
Monuments from the middle Ages include the 5–6th century Stara Mitropoliya ("old bishopric"), the 6th century church of the Virgin and the 11th century Nova Mitropoliya ("new bishopric"; also St Stephen) which continued to be embellished until the 18th century. In the 13th and 14th century a remarkable series of churches were built: St. Theodore, St. Paraskeva, St. Michael & St. Gabriel and St. John Aliturgetos.
