Developments in radiation therapy modalities offer alternative treatments for unresectable malignant tumors in the pelvis and trunk. However, poor vascularity as a result of radiation therapy makes the treated lesion susceptible to infection, and there are no established treatments for pelvic osteomyelitis with a large dead space after radiation therapy. The authors report 2 cases of sacral osteomyelitis after radiation therapy that were treated successfully with a gluteus maximus turnover flap. To create the flap, the distal portion of the lower third of the muscle was detached from the trochanter. The distal edge of the flap was turned toward the sacral defect and sewn to the remnant of the sacrum, which filled the dead space with the muscle bulk. A 68-year-old man with a recurrent sacral chordoma was treated with carbon ion radiation therapy; however, a sacral infection developed 5 months later. Debridement and a course of antibiotics could not control the infection and did not induce sufficient formation of granulation tissue in the large and deep dead space. The turnover flap with both gluteus maximus muscles cured the deep-seated infection and closed the wound. A 58-year-old woman had sacral osteoradionecrosis with infection. A turnover flap created with the left gluteus maximus muscle controlled the infection and closed the wound after the first operation, a V-Y flap, failed. This study showed that a gluteus maximus muscle turnover flap effectively controlled infectious lesions with large and deep dead space around the sacrum.