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The accessory sacroiliac (SI) joint is not rare and can be one of the causes of lower back pain. We present a case of a 35-year-old woman with chronic lower back pain. Conventional radiography was equivocal, and bone scintigraphy showed a focus of mildly increased uptake in left SI joint region. SPECT/CT revealed increased uptakes in the accessory articulations between both sacral alae and iliac bones, and confirmed the diagnosis of accessory SI joint. This case illustrates the SPECT/CT findings of the accessory SI joint.