Virtually all primary peritoneal carcinomas (PPCs) are of serous papillary type. We report an unusual histologic type of PPC composed of anaplastic giant cells, which exhibited an aggressive clinical course. A 72-year-old woman presented with lower abdominal pain. Computed tomography showed a diffuse omental thickening. The patient underwent an exploratory laparotomy with omentectomy, total hysterectomy, bilateral salpingo-oophorectomy, and appendectomy. Pathologic examination revealed extensive omental replacement by tumor but only superficial surface cortical involvement of both ovaries, a disease distribution consistent with a typical müllerian-derived PPC. However, this neoplasm was composed of diffuse anaplastic tumor giant cells, rather than serous carcinoma, which is the usual histologic type encountered in PPC. The patient died within 1 month after surgery. We report this unusual histologic variant of PPC to raise awareness that anaplastic giant cell carcinoma may arise in the pelvic peritoneum as a primary tumor.