BACKGROUND An accessory sacroiliac (SI) joint usually has little clinical significance. However, severe arthritic changes can cause chronic buttock or low back pain and can be misdiagnosed as another disease presenting with sacroiliitis such as ankylosing spondylitis (AS). METHODS A 33-year-old woman was diagnosed with AS due to chronic buttock pain and progressive sacroiliitis on plain X-ray and magnetic resonance imaging (MRI). Her buttock and low back pain gradually worsened despite proper treatment for AS. METHODS Computed tomography revealed an accessory SI joint with arthritic changes. METHODS Nonsteroidal anti-inflammatory drugs (NSAIDs) and restricted movement were prescribed. RESULTS The symptoms were controlled with NSAIDs, rest, and restriction of excessive movement. The medication could be stopped after the pain subsided. CONCLUSIONS An accessory SI joint can be a cause of chronic back pain and can be misdiagnosed as AS with sacroiliitis when progressive arthritic changes are observed. Therefore, additional imaging studies other than conventional X-ray or MRI may be required for accurate diagnosis.