Abstract: A safety valve for fitting into a compressed gas cylinder to terminate gas flow from the cylinder when a primary gas valve attached to the cylinder port of the cylinder is deflected or severed. The safety valve is independently supported in the cylinder port below the location of the primary gas valve. A threaded housing having a channel for the passage of gas to and from the cylinder is threaded into the cylinder port below the location of the primary gas valve. A poppet seat is provided in the housing. A tubular portion of the threaded housing extends up and fits snugly into the gas passage of the primary gas valve. A cylindrical housing having apertures for the passage of gas is attached to the threaded housing. A poppet is provided in the cylindrical housing, which is biased into the poppet seat by a helical spring. The poppet is held off of the poppet seat by means of a frangible element.