Although anatomical variation of the sciatic nerve and piriformis muscle at the greater sciatic notch is considered an important cause of piriformis syndrome, there are few reports on the surgical treatment of piriformis syndrome owing to specific anatomical variations of the sciatic nerve and piriformis muscle. In this report, we describe 2 cases of piriformis syndrome caused by a rare type C sciatic nerve variation that were surgically treated using the transgluteal approach. The first patient reported unremitting left hip and leg pain that occurred following blunt trauma to the hip. The second patient complained of chronic pain in the buttocks and right leg, which persisted even after the patient underwent lumbar fusion surgery. Severe sitting pain and sciatica are symptomatic indications for the diagnosis of piriformis syndrome. A rare "C" type sciatic nerve variation was observed on the affected side under magnetic resonance imaging. Transgluteal sciatic nerve decompression provided significant pain relief. If severe sciatic nerve deformation due to a rare sciatic nerve variation can be confirmed with typical findings of piriformis syndrome, the possibility that sciatic nerve entrapment may have occurred in this variation should be considered.