A safety device for a syringe constituted by a body, a needle carrier mounted at one end of the body, a piston that is axially movable in the body, and a piston plunger projecting from the body at its end opposite from the needle carrier and suitable for pushing the piston inside the body towards the needle carrier, the device comprising a sheath in which the syringe body can slide axially between an injection position in which the needle carrier is flush with one end of the sheath and a safe position in which the needle carrier is retracted into the sheath, a helical spring which is axially compressed when the syringe body is in the injection position, said spring then exerting on said body a force tending to cause it to slide into the safe position, retention means for retaining the syringe body in the injection position, and a release mechanism for releasing the retention means acting on said retention means when the piston plunger of the syringe has pushed the piston inside the body into the vicinity of the needle carrier, wherein the retention means for retaining the body of the syringe in the injection position in the sheath are constituted by at least one radially-movable resilient tab extending substantially longitudinally relative to the sheath and arranged to hold the spring in the compressed state even when the syringe body is not in the sheath.