Fuel injector

The invention relates to a fuel injector for a fuel injection system, in particular a common-rail injection system, comprising a nozzle body (1) and an injector body (2), which each enclose a high-pressure chamber (3, 4), in which a one-piece or multi-piece nozzle needle (5) for opening and closing at least one injection opening is accommodated in a lifting-movable manner, wherein the nozzle needle (5) is guided through a low-pressure chamber (7), which is sealed from the high-pressure chambers (3, 4) via at least one guide gap (8, 9) between the nozzle needle (5) and a body (6) bounding the low-pressure chamber (7) and/or a guide element (10, 11) supported thereon, and wherein the nozzle needle (5) has guide diameters (d o , d u ) of different sizes for forming a pressure stage (12) arranged in the low-pressure chamber (7). According to the invention, the guide diameter (d o ) located farther away with respect to the injection opening is smaller than the guide diameter (d u ), so that the nozzle needle (5) is partially force-relieved in the opening direction.