The present invention relates to a safety device (2) for a prefilled syringe (1), the syringe comprising a body (3), a needle carrier (4), a piston (6) movable inside the body, and a piston rod (7) suitable for pushing the piston inside the body towards the needle carrier. The device comprises a protective sheath (10) in which the syringe body can slide between an injection position and a safe position in which the needle is withdrawn inside the sheath, said device (2) comprising a resilient element (15) suitable for acting on the syringe to bring in into its safe position after injection, said resilient element presenting two axial ends and comprising at least two elastically-deformable arms extending between said two axial ends, one of the ends (20) being arranged so as to be capable of sliding freely relative to the piston rod (7) prior to injection, and/or one of the ends being arranged in such a manner as to be capable of being fixed to the protective sheath (10) in a position of adjustable height.