The Lockheed YO-3 "Quiet Star" was an American single-engined, propeller-driven monoplane that was developed for battlefield observation during the Vietnam War. It was designed to be as quiet as possible, and was intended to observe troop movements in near-silence during hours of darkness.

The YO-3A was designed to a U.S. Army specification of 1968, which called for an observation aircraft that would be acoustically undetectable from the ground when flying at an altitude of 1,200 feet at night. Lockheed was approached to produce such a design. In 1966 the company built a prototype QT1 "Quiet Thrust", also known as X-26B, using a modified Schweizer SGS 2-32 glider. This was abandoned for two prototypes of a two-seat version called the QT-2 "Prize Crew". The QT had a silenced engine and a propeller operating at subsonic tip speed for quiet operation.

Following operation trials with the QT-2 in Vietnam, 1968, a production version, designated the YO-3A was ordered by the Army. Based on the QT2's SGS 2-32 platform, the YO-3A was highly modified. It had a low-mounted wing, retractable main-wheel landing gear and a modified fuselage with tandem seating for a pilot in back and an observer in the front seat using a NVAP (Night Vision Aerial Periscope) and infrared illuminator. The YO-3A was powered by a 210 hp IO-360 engine driving a six-bladed, fixed-pitch propeller; the propeller was later changed to a wooden three-bladed constant-speed version.