The diocese was originally part of the Diocese of Thérouanne, which had been established in the 7th or 8th century. In 1553 Charles V besieged the city of 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 razed the city. In 1557, as a result of the war damage to its see, the diocese was abolished. This led to a reform of sees at the Council of Trent, Council of Trent, and the bishopric of Thérouanne was split between the Diocese of Saint-Omer, the diocese of Boulogne and the Diocese of Ypres. With this, Saint Martin's Church was elevated to cathedral status, as it became the see of the new diocese.
