In a gas-supply system for internal-combustion engines, a pressure-reducing valve is set between the reservoir of the gas and a distribution manifold or rail that feeds the gas to a series of electromagnetically controlled injectors associated to the various cylinders of the engine. The pressure-reducing valve has a piston member that faces a chamber set downstream of a restricted passage defined inside the body of the valve in communication between the inlet connector and the outlet connector. The piston member is pushed by thrust means towards a position in which it causes opening of an open/close element (valve head) that controls the communication between said restricted passage and the inlet connector. The piston member is preferably provided with a single seal gasket that prevents leakage of gas towards the outside through the sliding coupling of the piston member within the respective guide bushing. The thrust means to which the piston member is subject may be constituted by spring means, or else by a pilot-pressure signal, in the case where it is intended to obtain a variable reduction of the pressure according to the operating conditions of the system.