The presence of a supernumerary posterior atlantoaxial facet joint has not been reported in the literature.To describe a patient who presented with a unilateral anomalous posterior facet joint and discuss its possible embryological origin.Case report.A 35-year-old woman presented with severe neck pain, headaches, and tingling radiating down her left arm. Radiological studies showed an anomalous right C1-C2 facet joint causing dorsal compression of the spinal cord and cord compression secondary to a C5-C6 herniated disc. The patient underwent resection of the abnormal facet via a posterior approach followed by anterior cervical discectomy and fusion at C5-C6.The patient reported marked improvement in her neck pain and partial improvement in her left upper extremity numbness and tingling. Postoperative radiographs showed no abnormal motion at C1-C2.The authors hypothesize that, in this patient, the C1 and C2 sclerotomes may have fused abnormally posteriorly, followed by the initiation of events at this site that eventually led to the formation of a morphologically normal synovial joint in an abnormal location.