We identified the accessory sacroiliac joint on 13 (13%) of 100 CT scans of the pelvis and in nine (16%) of 56 dried skeletons. The joint is situated between the medial aspect of the posterior superior iliac spine and a rudimentary transverse tuberosity just lateral to the second sacral foramen. Some joints are true diarthrodial joints and are present at birth, but more commonly they are acquired fibrocartilaginous joints that result from the stress of weight-bearing. Our experience suggests that the accessory sacroiliac joint is not rare and that it is visible on CT scans in many patients.