A pressure regulator is disclosed which includes a valve housing with a sliding piston member defining inlet and outlet chambers. A valve seat in conjunction with the piston member forms a valve controlling fluid passing from the inlet to the outlet chamber. The piston member includes a piston and piston plate, the piston plate engaging the valve seat to control fluid flow. The diameters of the chambers and the piston are selected so that the inlet pressure causes no net force on the moving piston member, and does not contribute to either the opening or closing of the valve. On the outlet side, the piston plate is sealed with respect to the valve seat at a diameter which is less than that of the piston plate, such that the outlet pressure creates a net force urging the piston plate to engage the valve seat. The resulting unbalanced output force on the member is opposed by coil spring urging the member upward to unseat the plate from the valve seat. The output pressure is thereby regulated at a point where the outlet force equals the spring force. Alternate embodiments allowing selection of the spring force are disclosed.