Sacroiliac joint block can be performed for the diagnosis and treatment of sacroiliac joint dysfunction. Although sacroiliac joint block is a common procedure, complications have not been reported in detail. We report a case of iliacus pyomyositis and sacroiliac joint infection following a sacroiliac joint block. A 70-year-old female patient received sacroiliac joint blocks to relieve pelvic pain. The patient was admitted to the emergency room two days after the final sacroiliac joint block (SIJB) with the chief complaints of left pelvic pain corresponding to a visual analogue scale (VAS) score of 9 and fever. A pelvic MRI indicated a diagnosis of myositis. After 1 month of continuous antibiotic therapy, the patient's erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) level remained elevated. A 67Ga SPECT/CT was done. Abnormal uptake was seen at the left sacroiliac joint (SIJ), and septic sacroiliitis was suspected. The CRP normalized to 0.29 mg/dl and the ESR decreased to 60 mm/hr, and the patient had no fever after 57 days of antibiotic therapy. She was directed for follow up at an outpatient clinic.