The first recorded mention of Svendborg occurred in 1229 in a letter by Valdemar the Victorious, where he refers to the fortification, known as Svinæburgh. The explanations for this early form of the name vary. Some attribute the name to the excessive numbers of pigs (Danish: svin) in the area, others to large numbers of Harbour Porpoises (Danish: marsvin), while still others hold that the name simply implies that it is a fortress located by the sound.
In 1236, the Greyfriars monastery in Svendborg was established. The Greyfriars would be part of the city for the next 300 years, until the Protestant reformation in 1536. The ruins of the monastery were partly excavated in 2007.
In 1253, the city was granted market town privileges by King Christopher I.
During Medieval times the city was fortified with walls and moats. The defense system also included a small number of fortresses. Most historical facts about the medieval defense system, including the locations of fortifications, are disputed, as little archaeological evidence has been generated. In spite of this, it is a popular theory that the three towers in the coat of arms are the three fortifications ("Skattertårnet", "Kyseborg" and a third unnamed one).
During the time of the Protestant reformation and the Count's Feud in the 1530s, the citizens of Svendborg joined forces with the King. Ørkild Castle, located just east of Svendborg, was property of the bishop of Odense, who was less than popular among the citizens of the city. The tension resulted in the castle being seized and burned down by an angry mob in collaboration with the King's forces. The King's forces would later, after ending their north-going campaign on Funen, return to pillage and plunder Svendborg.
After 1536, Svendborg went through a brief period of progress, but it would not last for long. In the following 250 years, the city would have to face various setbacks in its development, such as plague, hostile
