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The S-IV was the second stage of the Saturn I rocket used by NASA for early flights in the Apollo program.

The S-IV was manufactured by the Douglas Aircraft Company and later modified by them to the S-IVB, a similar but distinct stage used on the Saturn IB and Saturn V rockets.

The S-IV stage was a large LOX/LH-fueled rocket stage used for the early test flights of the Saturn I rocket. It formed the second stage of the Saturn I and was powered by a cluster of six RL-10A-3 engines. Each one of the engines supplied of thrust for a total of about . The cryogenic LH ( liquid hydrogen) and LOX ( liquid oxygen) tanks were separated by a common bulkhead. The forward bulkhead of the LOX tank formed the aft bulkhead of the LH tank. This saved about ten tons of structural weight.