The invention relates to a safety device for an injection syringe, the injection syringe including a body having an injection needle fitted thereon, the device including a plunger rod including a distal plunger part slidably movable in the body of the syringe, and a proximal triggering part, configured to trigger the displacement of a protection sheath, from an injection position, wherein the needle is uncovered, towards a safety position, wherein the needle is covered by the sheath. The proximal triggering part is able to move relative to the distal plunger part once the distal plunger part is at the distal end of its stroke, to trigger the displacement of the protection sheath.