A cryogenic engine is a rocket engine that uses a cryogenic fuel and oxidizer, which are gases that have been liquefied and stored at very low temperatures. Cryogenics is the study of the production and behavior of materials at extremely low temperatures, below -150 degrees Celsius, to lift and place heavier objects in space. Cryogenic engines are the most prestigious rocket engine technology due to their design and operational complexity, and only a few countries have been able to develop them.

The major components of a cryogenic rocket engine include the combustion chamber, pyrotechnic initiator, fuel injector, fuel and oxidizer turbopumps, cryo valves, regulators, fuel tanks, and rocket engine nozzle. Cryogenic rocket engines are almost exclusively pump-fed, meaning that the propellants are fed to the combustion chamber using pumps.

The most widely used cryogenic fuel-oxidizer combination is liquid hydrogen (LH2) fuel and liquid oxygen (LOX) oxidizer, which are easily and cheaply available and produce a specific impulse of up to 450 seconds at an effective exhaust velocity of 4.4 kilometers per second. Cryogenic temperatures vary depending on the propellant, with liquid oxygen existing below −183 °C and liquid hydrogen below −253 °C.

Cryogenic engines are reaction engines that throw mass in one direction and rely on the reaction thrust in the opposite direction. They are more efficient and provide more thrust for every kilogram of propellant burned compared to solid and earth-storable liquid propellant rocket stages. However, cryogenic stages are technically very complex systems compared to solid or earth-storable liquid propellant stages due to their use of propellants at extremely low temperatures and the associated thermal and structural problems.