Mostaganem (mōsˌtägänĕmˈ) [key], city (1998 pop. 124,399), capital of Mostaganem prov., NW Algeria, a port on the Mediterranean Sea. It was founded in the 11th cent. and reached its commercial height in the late 16th cent. under the Turks. Its population had declined to about 3,000 when the French arrived in 1833. Wine, grain, meat, and wool are exported.