A cooked prime rib should reach an internal temperature of 125 to 130 degrees F at its thickest part. Let it sit covered in foil and the temperature will continue to increase. The outer layers of the roast will be medium well to medium, and be closer to medium rare as you get closer to the center.u003cbr/u003eRare 125 degrees Fu003cbr/u003eMedium rare 130 degrees F