The United States Army Air Service (USAAS) (also known as the "Air Service", "U.S. Air Service" and before its legislative establishment in 1920, the "Air Service, United States Army") was the aerial warfare service component of the United States Army between 1918 and 1926 and a forerunner of the United States Air Force. It was established as an independent but temporary branch of the U.S. War Department during World War I by two executive orders of President Woodrow Wilson: on May 24, 1918, replacing the Aviation Section, Signal Corps as the nation's air force; and March 19, 1919, establishing a military Director of Air Service to control all aviation activities. Its life was extended for another year in July 1919, during which time Congress passed the legislation necessary to make it a permanent establishment. The National Defense Act of 1920 assigned the Air Service the status of "combatant arm of the line" of the United States Army with a major general in command.

In France, the Air Service of the American Expeditionary Force, a separate entity under commanding General John J. Pershing that conducted the combat operations of U.S. military aviation, began field service in the spring of 1918. By the end of the war, the Air Service used 45 squadrons to cover 137 kilometers (85 miles) of front from Pont-à-Mousson to Sedan. 71 pursuit pilots were credited with shooting down five or more German aircraft while in American service. Overall the Air Service destroyed 756 enemy aircraft and 76 balloons in combat. 17 balloon companies also operated at the front, making 1,642 combat ascensions. 289 airplanes and 48 balloons were lost in battle.

The Air Service was the first form of the air force to have an independent organizational structure and identity. Although officers concurrently held rank in various branches, after May 1918 their branch designation in official correspondence while on aviation assignment changed from "ASSC" (Aviation Section, Signal Corps) to "AS, USA" (Air Service, United States Army). After July 1, 1920, its personnel became members of the Air Service branch, receiving new commissions. During the war its responsibilities and functions were split between two coordinate agencies, the Division of Military Aeronautics (DMA) and the Bureau of Aircraft Production (BAP), each reporting directly to the Secretary of War, creating a dual authority over military aviation that caused unity of command difficulties.

The seven-year history of the post-war Air Service was marked by a prolonged debate between adherents of airpower and the supporters of the traditional military services about the value of an independent Air Force. Airmen such as Brig. Gen. Billy Mitchell supported the concept. The Army's senior leadership from World War I, the United States Navy, and the majority of the nation's political leadership favored integration of all military aviation into the Army and Navy. Aided by a wave of pacifism following the war that drastically cut military budgets, opponents of an independent air force prevailed. The Air Service was renamed the Army Air Corps in 1926 as a compromise in the continuing struggle.