A glowplug (alternatively spelled as glow plug or glowplug) is a heating device used to aid in starting diesel engines. In cold weather, highspeed diesel engines can be difficult to start because the mass of the cylinder block and cylinder head absorb the heat of compression, preventing ignition. Prechambered engines use small glowplugs inside the prechambers. Directinjected engines have these glowplugs in the combustion chamber.

The glowplug is a pencilshaped piece of metal with a heating element at the tip. This heating element, when electrified, heats due to its electrical resistance and begins to emit light in the visible spectrum, hence the term glowplug. The visual effect is similar to the heating element in a toaster. The fuel injector spray pattern then impinges directly upon the hot tip of the glow plug during the injection of fuel at top dead center. This ignites the fuel even when the engine is insufficiently hot for normal operation, which reduces the cranking time required to start the engine.