A tripropellant rocket is a rocket that uses three propellants, as opposed to the more common bipropellant rocket or monopropellant rocket designs, which use two or one propellants, respectively. Tripropellant systems can be designed to have high specific impulse and have been investigated for single-stage-to-orbit designs. While tripropellant engines have been tested by Rocketdyne and NPO Energomash, no tripropellant rocket has been flown.

There are two different kinds of tripropellant rockets. One is a rocket engine which mixes three separate streams of propellants, burning all three propellants simultaneously. The other kind of tripropellant rocket is one that uses one oxidizer but two fuels, burning the two fuels in sequence during the flight.

Simultaneous tripropellant systems often involve the use of a high energy density metal additive, like beryllium or lithium, with existing bipropellant systems. In these motors, the burning of the fuel with the oxidizer provides activation energy needed for a more energetic reaction between the oxidizer and the metal. While theoretical modeling of these systems suggests an advantage over bipropellant motors, several factors limit their practical implementation, including the difficulty of injecting solid metal into the thrust chamber; heat, mass, and momentum transport limitations across phases; and the difficulty of achieving and sustaining combustion of the metal.

In the 1960s, Rocketdyne test-fired an engine using a mixture of liquid lithium, gaseous hydrogen, and liquid fluorine to produce a specific impulse of 542 seconds, likely the highest measured such value for a chemical rocket motor. Despite the high specific impulse, the technical difficulties of the combination and the hazardous nature of the propellants precluded further development.

In sequential tripropellant rockets, the fuel is changed during flight, so the motor can combine the high thrust of a dense fuel like kerosene early in flight with the high specific impulse of a lighter fuel like liquid hydrogen (LH2) later in flight. The result is a single engine providing some of the benefits of staging.

For example, injecting a small amount of liquid hydrogen into a kerosene-burning engine can yield significant specific impulse improvements without compromising propellant density. This would have been demonstrated by the RD-701, theoretically capable of a specific impulse of 415 seconds in vacuum (higher than the pure LH2/LOX RS-68), where a pure kerosene engine with a similar expansion ratio would achieve 330–340 seconds.

Although liquid hydrogen delivers the largest specific impulse of the plausible rocket fuels, it also requires huge structures to hold it due to its low density. These structures can weigh a lot, offsetting the light weight of the fuel itself to some degree, and also result in higher drag while in the atmosphere. While kerosene has lower specific impulse, its higher density results in smaller structures, which reduces stage mass, and furthermore reduces losses to atmospheric drag. In addition, kerosene-based engines generally provide higher thrust, which is important for takeoff, reducing gravity drag. So in general terms there is a "sweet spot" in altitude where one type of fuel becomes more practical than the other.

Traditional rocket designs use this sweet spot to their advantage via staging. For instance the Saturn Vs used a lower stage powered by RP-1 (kerosene) and upper stages powered by LH2. Some of the early Space Shuttle design efforts used similar designs, with one stage using kerosene into the upper atmosphere, where an LH2 powered upper stage would light and go on from there. The later Shuttle design is somewhat similar, although it used solid rockets for its lower stages.