The sacroiliac (SI) joint is a critical structure located at the base of the spine, where the sacrum (the triangular bone at the bottom of the spine) meets the ilium (part of the pelvic bone). This joint plays a key role in transferring weight and forces between the upper body and the legs. Although the SI joint has limited movement, it provides stability and support to the pelvis and spine. Dysfunction or inflammation of the SI joint can cause lower back and buttock pain, often referred to as sacroiliitis. Treatment typically includes physical therapy, medications, joint injections, and in severe cases, surgical interventions. Proper diagnosis and management are essential for relieving pain and restoring function.