Sacroiliac (SI) joint pain occurs where the spine meets the pelvis, causing deep aching pain in the lower back or buttock, usually on one side. The pain can spread into the hip or groin and often worsens with activities like standing up from a chair, climbing stairs, or twisting. It commonly results from inflammation, arthritis, pregnancy-related ligament changes, or muscle imbalance around the pelvis. Because symptoms can mimic sciatica or lumbar spine problems, diagnosis relies on physical exam maneuvers and sometimes an injection to confirm the source. Treatment usually includes physical therapy, anti-inflammatory medications, and occasionally targeted injections.