Introduction and importance Myositis ossificans of the hip is a rare entity caused by trauma and neurological conditions which lead to lamellar bone formation around the joint. Case presentation We present a 47 years old patient with myositis ossificans of the left and the piriformis muscle following Guillain Barré syndrome causing sciatic nerve palsy (Piriformis syndrome). Clinical discussion Clinical assessment revealed global limitation of the left hip movements and palpable bony mass on the lateral aspect of the left hip. Radiographs and computed tomography showed extensive myositis ossificans of the left hip and non-bridging calcification noted on the asymptomatic right side. During initial medical management and physiotherapy patient developed sciatic nerve palsy due to piriformis syndrome and ankylosis of the hip on the left side. Surgical exploration of the sciatic nerve and debridement of the hip. The left hip was found ankylosed and not salvageable. Uncemented total hip arthroplasty was carried out in the session. The patient recovered completely from sciatic nerve palsy and regained the range of motion of the hip. He is under follow-up for the recurrence of myositis ossificans. Conclusion Guillain-barré syndrome causing piriformis syndrome is a rare entity. Extensive myositis ossificans causing sciatic nerve palsy is even rarer. An awareness of this entity, early detection, and intervention of this condition may help to preserve the native hip.