The serratus posterior inferior muscle, also known as the posterior serratus muscle,^{[citation needed]} is a muscle of the human body.

The muscle is situated at the junction of the thoracic and lumbar regions.^[1] It has an irregularly quadrilateral form, broader than the serratus posterior superior muscle, and separated from it by a wide interval.

It arises by a thin aponeurosis from the spinous processes of the lower two thoracic and upper two or three lumbar vertebrae.^[1]

Passing obliquely upward and lateralward, it becomes fleshy, and divides into four flat digitations.^[1] These are inserted into the inferior borders of the lower four ribs, a little beyond their angles.^[1]

The thin aponeurosis of origin is intimately blended with the thoracolumbar fascia, and aponeurosis of the latissimus dorsi muscle.^{[citation needed]}

The serratus posterior inferior draws the lower ribs backward and downward to assist in rotation and extension of the trunk.^[1] This movement of the ribs may also contribute to inhalation and forced expiration of air from the lungs.^[2]

• Serratus anterior muscle
• Serratus posterior superior muscle