A safety support device for a syringe comprises a support sheath (16) for the body (10) of the syringe and an inner sheath (18) suitable for sliding relative to the support sheath between a waiting, retracted position and the extended position for protecting the needle (14) of the syringe. The device has means for holding the syringe body (10) relative to the support sheath (16) and retaining means (30, 32, 22) suitable for being activated to retain the inner sheath in the waiting, retracted position inside the support sheath and for being deactivated by the piston (12) of the syringe to allow the inner sheath (18) to extend into its protective, extended position.