Apollo 4, also known as AS-501, was the first, uncrewed, flight in the United States's Apollo program, and the first test of the Saturn V launch vehicle, the rocket that would be used to send astronauts to the Moon. The space vehicle was assembled in the Vehicle Assembly Building, and was the first to be launched from Kennedy Space Center in Florida, ascending from Launch Complex 39, where facilities built specially for the Saturn V had been constructed.

Apollo 4 (November 9, 1967), also known as AS-501, was the first, un-crewed, flight in the United States Apollo program, and the first test of the Saturn V launch vehicle, the rocket that would be used to send astronauts to the Moon. The space vehicle was assembled in the Vehicle Assembly Building, and was the first to be launched from Kennedy Space Centre (KSC) in Florida, ascending from Launch Complex 39, where facilities built specially for the Saturn V had been constructed.

Apollo 4 was an "all-up" test, meaning all rocket stages and spacecraft were fully functional on the initial flight, a first for NASA. It was the first time the S-IC first stage and S-II second stage flew. It also demonstrated the S-IVB third stages' first in-flight restart. The mission used a Block I command and service module (CSM) modified to test several key Block II revisions, including its heat shield at simulated lunar-return velocity and angle.

The original launch date was planned for late 1966, but was delayed to November 9, 1967, due to a myriad of problems with various elements of the spacecraft, and difficulties during pre-flight testing. Also contributing to the delays was the need for additional inspections following the Apollo 1 fire that killed the first Apollo crew in January 1967. These issues delayed the flight through much of 1967.

The mission splashed down in the Pacific Ocean slightly less than nine hours after launch, having achieved its objectives. NASA deemed the mission a complete success, as it proved the Saturn V worked, which was an important step towards achieving the main objective: landing astronauts on the Moon and bringing them back safely, before the end of the 1960s.