The psoas major is a lengthy fusiform (or ‘spindle-shaped’) muscle that can be found on the side of the lumbar region of the vertebral column. It is also adjacent to the brim of the lesser pelvis area. It connects the iliacus muscle to the iliopsosas. In some individuals, the muscle is assisted by the psoas minor.

The muscle is divided into two parts; it has both a superficial part and a deep part. The deep part starts at the transverse process of the lumbar vertebrae. The superficial part starts at the lateral surface of the end of the thoracic vertebrae, the lumbar vertebrae, and nearby invertebral discs. The lumbar plexus is what separates the superficial part from the deep part.

The psoas major assists with the flexion and external rotation of the hip joint. It is also considered part of the group of muscles that are known as the hip flexors. If this muscle tightens, the action may result in lower back pain.