Boulogne-sur-Mer (also Boulogne), city in northern France, in the Pas-de-Calais Department of the Nord-Pas-de-Calais administrative region, on the English Channel. It is France's leading fishing port and a terminus of the cross-channel ferry service. Its products include processed fish, boats, and building materials. The city is divided into the old, fortified town perched on top of a hill, and a newer commercial and industrial quarter at its foot. Notable structures include a castle (13th century) and the Cathedral of Notre Dame (19th century, but built above a medieval Gothic crypt), both inside the old town.

The Roman port of Gesoriacum (later called Bononia) was situated here, from where the Romans sailed to invade Britain. In 1803-1805 Napoleon assembled a fleet here for a planned invasion (subsequently cancelled) of England with his Grande Armée. The British used Boulogne-sur-Mer as a key base in World War I, and it was a German submarine base in World War II, when most of the lower town was destroyed. Population 45,100 (2005 estimate).