Mortagne-sur-Gironde is a commune in the Charente-Maritime department in southwestern France. Bordering the banks of the Gironde estuary, this small town was for centuries a principality, a title it acquired after the Hundred Years War, but ended up losing by escheat. Being an important military stronghold, it also became a leading port in the 18th century, with the Gironde port bein ranked third in the middle of the 20th century, before declining gradually.