A settlement on the site of today's town was founded in the Roman times. Initially referred to as Gesoriacum or Portus Itius, in the 4th Century it received the permanent name of Bononia and became firmly established as a prominent Roman trading and military post in Gallia and an important link with the Empire. Emperor Claudius chose the town for the base of his army during the Roman invasion of Britain which took place in AD 43. Until the end of the 3rd Century, Bononia remained a major stronghold of the Classis Britannica, the naval fleet of the ancient Rome.

In the following centuries, the ancient Roman walls served as a shelter for a Medieval town which flourished within. At that time, the bustling town centre was dominated by a beautiful castle of Seigneurs, the counts of Boulogne. It was also this period that saw the construction of an early version of the most recognisable symbol of Boulogne, the Basilica of Notre-Dame de Boulogne. The imposing  church still holds the remains of the legendary figure of Our Lady of the Sea, a popular object of pilgrimages in the Middle Ages.

The Basilica of Notre-Dame de Boulogne is considered to express the soul of this seaside town because it has seen the best and the worst of its times. Until the French Revolution, the church – like the town – flourished, playing host to the 1308 wedding of Edward II of England and Isabella of France. However, during the revolution, worship in the church was prohibited and the building itself sold to merchants from outside of the province. Gradually, they demolished the greater part of the original contruction and in 1793, burned its famous statue, of which only a small portion of the hand piece remains.