The 638th Aero Squadron was a Air Service, United States Army unit that fought on the Western Front during World War I.
The squadron was assigned as a Day Pursuit (Fighter) Squadron as part of the 5th Pursuit Group, Second United States Army. Its mission was to engage and clear enemy aircraft from the skies and provide escort to reconnaissance and bombardment squadrons over enemy territory.
The squadron was never fully organized, and with Second Army's planned offensive drive on Metz cancelled due to 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 July 1919 and was demobilized.
The squadron was never re-activated, and there is no United States Air Force or Air National Guard squadron that carries its lineage and history.