Boulogne-sur-Mer is a city in northern France, in the Pas-de-Calais département of which it is a sous-préfecture. It is located by the English Channel. Population (1999): 45,508.

Originally named Gesoriacum , by the 4th century Boulogne was known to the Romans as Bononia and served as the major port connecting the rest of the empire to Britain. The emperor Claudius used this town as his base for the Roman invasion of Britain. In the Middle Ages it was the centre of a namesake county.

In the year 1905 the First Esperanto Universal Congress was held in Boulogne-sur-Mer. L. L. Zamenhof, the creator of Esperanto, was among the attendees.