The iliacus (also iliacus muscle, latin: musculus iliacus) is a flat skeletal muscle of the pelvis, filling the iliac fossa and together with psoas major forming the iliopsoas muscle.
The iliacus originates from the iliac fossa.
The iliacus together with the psoas major inserts onto the lesser trochanter of the femur.
The main functions of the iliacus muscle are flexion of the hip joint and external rotation of the thigh.
The iliacus is innervated by the muscular branches of the lumbar plexus.