A 26-year-old nonleukemic woman presented with lumbosacral granulocytic sarcoma manifesting as progressive low back pain and numbness of her left lower leg persisting for 3 months. Physical examination revealed hypesthesia within the left S1 area of the sensory dermatome, decreased Achilles tendon reflex in the left lower extremity, and walking impairment due to severe pain in her left hip and leg. Magnetic resonance imaging confirmed an extradural mass in the spinal canal at the L5-S2 levels with invasion to the pelvis from the left sacral foramen. Positron emission tomography with [18F]fluorodeoxyglucose (FDG-PET) showed hyperaccumulation indicating malignant tumor. Baseline laboratory data were normal. Decompressive laminectomy and tumor removal were performed. Histological examination identified granulocytic sarcoma. Bone marrow involvement was absent. She underwent adjuvant chemotherapy and radiotherapy, resulting in reduced residual lesion and neurological improvement. Immediate diagnosis and adequate systematic treatment are recommended for spinal granulocytic sarcoma in nonleukemic patients to prevent or delay progression to leukemia. The importance of immunohistochemical staining in the differential diagnosis from other types of spinal tumor, and the efficacy of FDG-PET for evaluation of the treatment are also emphasized.