The invention relates to a safety device (12) for an injection syringe (14), the injection syringe comprising a body (16) having an injection needle (18) fitted thereon, the device (12) comprising a plunger rod (22, 28) including a distal plunger part (28D) slidably movable in the body (16) of the syringe, and a proximal triggering part (28P), configured to trigger the displacement of a protection sheath (24), from an injection position, wherein the needle (18) is uncovered, towards a safety position, wherein the needle (18) is covered by the sheath (24). The proximal triggering part (28P) is able to move relative to the distal plunger part (28D) once the distal plunger part is at the distal end of its stroke, to trigger the displacement of the protection sheath (24).
