The 139th Aero Squadron was a United States Army Air Service 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 2d Pursuit Group, First 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. It also attacked enemy observation balloons, and performed close air support and tactical bombing attacks on enemy forces along the front lines.