A pressure regulator for a fuel cell system may include: a housing having an inlet port into which gas is introduced, an outlet port through which gas is discharged, and an regulated pressure chamber connected to an outside through the outlet port; a piston moving up and down in the housing, having a main flow path penetrating through an inside of the housing and a pressure acting portion applying a pressure of gas in the regulated pressure chamber of the housing, and selectively communicating between the inlet port and the regulated pressure chamber of the housing through the main flow path as the piston moves up and down; and a spring provided in the housing to elastically support the pressure acting portion of the piston on an opposite side of the regulated pressure chamber.