Giant cell tumor (GCT) is a locally aggressive tumor but with benignity features, representing approximately 18% of non-malignant bone tumors in European countries, with slight female predominance. Malignancy in GCT is rare, about <2% of cases and is more common at older ages. Is known that usually occurs at the epiphyses of long bones, but extremely rare may have another location, such as the pelvic bone. An atypical location - the posterior iliac bone, found at a 34-year-old male -, is the case report we studied and described. Starting from the patient's complains, like a mass in the left buttock region described as "recently appeared", firm, not-mobile, with no distinctive borders and no tenderness at palpation, and a recent history trauma, multiple investigations have been performed, which have highlighted an osteolytic lesion, close to the sacroiliac joint, only with infiltration of the gluteal, iliac and paravertebral muscles. The treatment of choice was hemipelvectomy, with wide tumoral resection, and selective embolization of the nutrient vessels 24 hours prior to the surgical procedure. At two years postoperative, we found a good functional result and the computed tomography (CT) scan revealed no signs of recurrence.