is a device for injecting pressurized grease into a bearing; the bearing receives the grease through a spring-loaded ball-check valve that lets the grease in, but prevents it from running back out. A grease gun contains a reservoir for grease, a nozzle that fits tight against the fitting or clamps on to it, and a means for applying pressure to the grease. A grease gun has advantages over a grease cup: it can handle a harder grease, and the pressure can drive the fresh charge of grease into very tight clearances and flush out old grease that may have become contaminated with foreign matter.

Also see high-pressure-injection injury.