A needle-less syringe adapter has a tube, a resilient valve, a piercing member and an injection member. The tube has a body has a cavity and an outlet hole. The resilient valve is mounted in the cavity and has a chamber, a top slit and a bottom slit. The piercing conduit is mounted in the chamber, is driven to move when the resilient valve is compressed, selectively extends out through the top and bottom slits and has a through hole defined through the piercing conduit. The injection member is mounted on the tube and is selectively sealed by the resilient valve. The needle-less syringe adapter has excellent hermetical characteristics to prevent air infiltration and backflow of bloody and liquid medicine in a patient's blood vessel.