A monopropellant rocket (or "monochemical rocket") is a rocket that uses a single chemical as its propellant.^{[contradictory]} Monopropellant rockets are commonly used as small altitude and trajectory control rockets in satellites, rocket upper stages, crewed spacecraft, and spaceplanes.

The simplest monopropellant rockets depend on the chemical decomposition of a storable propellant after passing it over a catalyst bed. The power for the thruster comes from the high pressure gas created during the decomposition reaction that allows a rocket nozzle to speed up the gas to create thrust.

The most commonly used monopropellant is hydrazine (N₂H₄, or H₂N−NH₂), a compound unstable in the presence of a catalyst and which is also a strong reducing agent. The most common catalyst is granular alumina (aluminum oxide, Al₂O₃) coated with iridium. These coated granules are usually under the commercial labels Aerojet S-405 (previously made by Shell) or W.C. Heraeus H-KC 12 GA (previously made by Kali Chemie). There is no igniter with hydrazine. Aerojet S-405 is a spontaneous catalyst, that is, hydrazine decomposes on contact with the catalyst. The decomposition is highly exothermic and produces a 1,000 °C (1,830 °F) gas that is a mixture of nitrogen, hydrogen and ammonia. The main limiting factor of the monopropellant rocket is its life, which mainly depends on the life of the catalyst. The catalyst may be subject to catalytic poisoning and catalytic attrition which results in the catalyst failure. Another monopropellant is hydrogen peroxide, which, when purified to 90% or higher concentration, is self-decomposing at high temperatures or when a catalyst is present.

Most chemical-reaction monopropellant rocket systems consist of a fuel tank, usually a titanium or aluminium sphere, with an ethylene-propylene rubber container or a surface tension propellant management device filled with the fuel. The tank is then pressurized with helium or nitrogen, which pushes the fuel out to the motors. A pipe leads from the tank to a poppet valve, and then to the decomposition chamber of the rocket motor. Typically, a satellite will have not just one motor, but two to twelve, each with its own valve.

The attitude control rocket motors for satellites and space probes are often very small, 25 mm (0.98 in) or so in diameter, and mounted in groups that point in four directions (within a plane).

The rocket is fired when the computer sends direct current through a small electromagnet that opens the poppet valve. The firing is often very brief, a few milliseconds, and — if operated in air — would sound like a pebble thrown against a metal trash can; if on for long, it would make a piercing hiss.

Chemical-reaction monopropellants are not as efficient as some other propulsion technologies. Engineers choose monopropellant systems when the need for simplicity and reliability outweigh the need for high delivered impulse. If the propulsion system must produce large amounts of thrust, or have a high specific impulse, as on the main motor of an interplanetary spacecraft, other technologies are used.

A concept to provide low Earth orbit (LEO) propellant depots that could be used as way-stations for other spacecraft to stop and refuel on the way to beyond-LEO missions has proposed that waste gaseous hydrogen—an inevitable byproduct of long-term liquid hydrogen storage in the radiative heat environment of space—would be usable as a monopropellant in a solar-thermal propulsion system. The waste hydrogen would be productively utilized for both orbital station-keeping and attitude control, as well as providing limited propellant and thrust to use for orbital maneuvers to better rendezvous with other spacecraft that would be inbound to receive fuel from the depot.