Ossification of sacrospinous ligament induces a great risk for maintaining the stability of the pelvis. The sacrospinous ligament, along with the sacrotuberous ligament, plays a distinct role in the sacroiliac joint and pubic symphysis stability. The ossification may cause compression of neurovascular structure traversing through the greater and lesser sciatic foramen. Here we report a case of unilateral heterogenous ossification of the left sacrospinous ligament causing sciatic nerve compression and sciatic pain. A 22-year-old Bangladeshi woman, mother of one child, presented with complaints of pain in the lower back, left buttock and back of the upper thigh. Clinical examination and investigations revealed a diagnosis of the partially ossified sacrospinous ligament with sciatic nerve compression. Total excision of heterotrophic calcification and partial excision of left sacrospinous ligament through posterior approach by a left paramedian incision over the lower back was performed under general anaesthesia. On outpatient follow-up visits at 2 weeks and 6 weeks post-surgery, complete disappearance of pain was observed, and the patient was able to return to regular productive life activity. In this report, we presented a rare case of ossified sacrospinous ligament causing sciatic nerve compression with unknown etiology. The surgical approach performed, total excision of heterotrophic calcification and partial excision of left sacrospinous ligament through the posterior approach helped to preserve the pelvic stability with a good clinical outcome.