The invention relates to an injector for a fuel injection system of an internal combustion engine, particularly in a motor vehicle, with an injector body which has a pressure booster section and a needle section. At least one injection hole is provided in the needle section. A nozzle needle which has an adjustable stroke is disposed in the needle section for controlling an injection of fuel through the at least one injection hole. A pressure booster is provided for increasing a fuel injection pressure relative to a system pressure. For this purpose, the pressure booster has a double-diameter or stepped piston, a control rod, and a pressure booster bottom which together form the boundary of a coupling chamber. A coupling path extends into the control rod and connects the coupling chamber, over a valve device which is located outside or within the injector, to a high-pressure supply of fuel.