Roer [?o.] was a department of the French First Republic and later First French Empire in present-day Germany and the Netherlands. It was named after the river Roer (Rur), which flows through the department. It was formed in 1795, when the Southern Netherlands and the left bank of the Rhine were occupied by the French. The department was formed from the duchies of Jülich and Cleves, the part of the Archbishopric of Cologne left of the Rhine, the Free City of Aachen, the Prussian part of the duchy of Guelders and some smaller territories. In 1805 the city of Wesel was added to the department. The capital was Aix-la-Chapelle (Aachen).
Its population in 1812 was 631,094.
After Napoleon was defeated in 1814, the department was divided between the United Kingdom of the Netherlands (left bank of the Meuse and a strip along its right bank including Gennep, Tegelen and Sittard, in present-day Dutch Limburg) and the Kingdom of Prussia (Province of Jülich-Cleves-Berg, now part of North Rhine-Westphalia, Germany).