AS-203 (or SA-203) was an unmanned flight of the Saturn IB rocket on July 5, 1966. It carried no Apollo Command/Service Module spacecraft, as its purpose was to verify the design of the S-IVB rocket stage restart capability that would later be used in the Apollo program to boost astronauts from Earth orbit to a trajectory towards the Moon. It successfully achieved its objectives, but the stage was inadvertently destroyed after four orbits.

The purpose of the AS-203 flight was to investigate the effects of weightlessness on the liquid hydrogen fuel in the S-IVB-200 second-stage tank. The lunar missions would use a modified version of the S-IVB-500 as the third stage of the Saturn V launch vehicle. This called for the stage to fire briefly to put the spacecraft into a parking Earth orbit, before restarting the engine for flight to the Moon. In order to design this capability, engineers needed to verify that the anti-slosh measures designed to control the hydrogen's location in the tank were adequate, and that the fuel lines and engines could be kept at the proper temperatures to allow engine restart.

In order to keep residual propellants in the tanks on orbit, there would be no Apollo Command/Service Module payload as there was on AS-201 and AS-202. This was replaced by an aerodynamic nose cone. Also, the full load of liquid oxygen oxidizer was shorted slightly so that the amount of hydrogen remaining would approximate that of the Saturn V parking orbit. The tank was equipped with 88 sensors and two TV cameras to record the fuel's behavior.

This was also the first flight of a new type of Instrument Unit that controlled the Saturn rockets during launch, and also the first launch of a Saturn IB from Pad 37B.

In the spring of 1966, the decision was made to launch AS-203 before AS-202, as the CSM that was to be flown on AS-202 was delayed. The S-IVB stage arrived at Cape Kennedy on 6 April 1966; the S-IB first stage arrived six days later, and the Instrument Unit came two days after that.

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