At the time of the Gauls, Tarwanna or Tervanna was the capital of the Belgian tribe of the Morini. After the Romans conquered Gaul, they too made the city the capital of the Civitas Morinorum district.
In the 7th century, probably around 639, Saint Audomar (Saint Omer) established the bishopric of Terwaan or Terenburg, the diocese of Thérouanne, which during the Middle Ages controlled a large part of the left bank of the river Scheldt. Territorially it was part of the county of Artois which belonged to the county of Flanders.
Thanks to that ecclesiastical control of some of the most prosperous cities north of the Alps, like Arras and Ypres, the bishopric was able to build a cathedral which was at the time the largest in France.
The town was captured by the Emperor Maximilian and Henry VIII from the French in 1513 after the battle of the Spurs. In 1553 Charles V besieged Thérouanne, then a French enclave in the Holy Roman Empire, in revenge for a defeat by the French at Metz. After he captured the city he ordered it to be razed, the roads to be broken up, and the area to be ploughed and salted. Only a small commune which lay outside the city walls, then named Saint-Martin-Outre-Eaux, was left standing, and later (probably around 1800) took over the name Thérouanne. Part of the portal of the cathedral was acquired by Saint-Omer; a colossal statue of Christ is all that is left of it these days.