The entrapment of pudendal nerve generally results with severe pain in the perineum. Although the relationship between sacrotuberous ligament (STL) and pudendal nerve entrapment (PNE) syndrome is well known, to the best of our knowledge, there has been no previously reported case of fully ossified STL, presenting with PNE syndrome. We present the case of a 61-year-old male with complaints of pain and burning on perineum. There was no history of trauma or other predisposing factors. The pain relief by local anesthetic blockage of the pudendal nerve supported the diagnosis of PNE syndrome. The plain radiograph revealed complete ossification of STL. Three-dimensional computed tomography clearly demonstrated the completely ossified STL. The ossified STL was totally excised with transgluteal incision. Ten days after surgery the patient’s complaints were particularly regressed and 1 month after surgery the patient was completely healed. In conclusion, completely ossified STL is a very rare cause of PNE syndrome, but it must be taken into consideration in the differential diagnosis of perineal ache. Plain radiography, anamnesis, and physical examination are sufficient for diagnosis. Three-dimensional computed tomography is useful for preoperative planning. Surgical treatment with transgluteal approach is a good choice for treatment.