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A thermojet is a rudimentary type of jet engine. At its heart is an ordinary piston engine, but instead of this driving a propellor, it drives a compressor. The compressed air is channelled into a combustion chamber, where fuel is injected and ignited. The high temperatures generated by the combustion cause the gasses in the chamber to expand. These gasses escape at high pressure from the exhaust of the engine, creating a reactive force that drives the engine.

These engines are reasonably inefficient because of the comparatively poor power-to-weight ratios of piston engines when compared to gas turbines. In fact, when such engines have been constructed, the aircraft designers would have achieved better results had they simply mounted a propellor on the engine instead. Nevertheless, the concept actually works and has been demonstrated in a number of different aircraft.

Thermojet research was abandoned at the end of World War II as the turbojet was demonstrably a far more practical solution to jet power.