A MARINE RISER CONNECTOR FOR UNDERWATER WELL HEAD EQUIPMENT, THE RISER CONNECTOR HAVING A PIN MEMBER PROVIDED WITH A GROOVE FOR RECEIVING A RESILIENT LOCK RING CARRIED BY A BOX MEMBER. THE LOCK RING HAS OVERLAPPED ENDS AND IS CENTRALIZED IN A GROOVE IN THE BOX BY SPRING-LOADED CENTRALIZER PINS. THE LOCK RING IS ACTUATABLE INTO LOCKING ENGAGEMENT WITHIN THE GROOVE IN THE PIN BY ACTUATOR SCREWS HAVING LOCK RING ENGAGING PADS, AND LOCK PLATES ARE PROVIDED TO PREVENT ROTATION OF THE ACTUATOR SCREWS EXCEPT WHEN THEY ARE ENGAGED BY A DRIVING TOOL.