The 91st Aero Squadron was a unit of the Air Service, United States Army that fought on the Western Front during World War I. The 91st was one of the first five American flying squadrons to reach France, arriving at Chaumont Hill 402 Aerodrome on 15 November 1917.

The squadron was assigned as an Army Observation Squadron, performing long-range, strategic reconnaissance over the entire length of the United States First Army sector of the Western Front in France. After the 1918 Armistice with Germany, the squadron was assigned to the United States Third Army as part of the Occupation of the Rhineland in Germany. It returned to the United States in June 1919