Rouen is a city on the River Seine, in northwestern France. It is the prefecture of the region of Normandy and the department of Seine-M…Rouen is a city on the River Seine, in northwestern France. It is the prefecture of the region of Normandy and the department of Seine-Maritime. Formerly one of the largest and most prosperous cities of medieval Europe, the population of the metropolitan area is 702,945. People from Rouen are known as Rouennais.