The iliolumbar ligament, which has five bands, is one of three vertebra-pelvic ligaments responsible for stabilizing the spine where its lumbar section meets the sacral section in the pelvis. It does this along with the sacrospinous and sacrotuberous ligaments. This ligament is essential to restraining movement of the lumbosacral junction in the lower back.

The iliolumbar ligament passes through the transverse process at the side of the fifth lumbar vertebra and down to the iliac crest’s inner lip on the posterior (back) side. The iliac crest is the top, outer portion of the ilium, the largest of the pelvic bones. The iliolumbar ligament forms a thickened, lower border for two thoracolumbar fascia (connective tissue) layers.

There are six ligaments in the lumbar spine region. Assisted by the lateral lumbosacral ligament, the iliolumbar ligament strengthens the lumbosacral joint, which connects the end of the lumbar spine with the beginning of the sacral section of the spine. Supraspinous, interspinous, ligamentum flavum, anterior longitudinal, and posterior longitudinal ligaments also play a role in strengthening vertebral joints.

This ligament stabilizes the connection between the pelvis and lower back, limiting trunk side-flexion. When this ligament becomes injured, bending to one side becomes painful in the sacroiliac joint area, though injuries to this ligament are rare. The sacroiliac joint area can be approximately located in some people by dimples in their lower back.