An adult patient presented with acute severe pelvic and low-back pain. Evaluations with CT and MRI were negative/inconclusive. F-18 FDG PET/CT localized the site of pathology in right sacroiliac-joint by demonstrating hypermetabolic activities conformal with the joint's outline, signifying infectious/inflammatory sacroiliitis. Blood culture was positive for Group B streptococcus (GBS). Antibiotic treatment was started within 24 hours from onset of symptoms, and there was almost immediate partial pain relief. GBS sacroiliitis is a rare form of septic sacroiliitis requiring prompt diagnosis and urgent treatment. This is the first report of F-18 FDG PET/CT for diagnosing GBS sacroiliitis in adults with no predisposing factors.