Psoas major is a long fusiform muscle located on the side of the lumbar region of the vertebral column and brim of the lesser pelvis. It joins the iliacus muscle to form the iliopsoas, and attaches into the top of the thigh (femur). In less than 50 percent of human subjects the psoas major is accompanied by the psoas minor. As part of the iliopsoas, psoas (major) contributes to flexion and external rotation in the hip joint. On the lumbar spine, unilateral contraction bends the trunk laterally, while bilateral contraction raises the trunk from its supine position.

It forms part of a group of muscles called the hip flexors, whose action is primarily to lift the upper leg towards the body when the body is fixed or to pull the body towards the leg when the leg is fixed.

For example, when doing a situp that brings the torso (including the lower back) away from the ground and towards the front of the leg, the hip flexors (including the iliopsoas) will flex the spine upon the pelvis.

Tightness of the psoas can result in lower back pain by compressing the lumbar discs.