Subcostalis is one of the muscles of the thoracic wall. On each side, it is best developed between ribs six and twelve.
It arises from the internal border, lateral to the angle, of the superior rib in each intercostal space. The muscle slopes obliquely both inferiorly and dorsally. It inserts into the internal surface of the ribs one to three intercostal spaces caudally near to the angle of the rib.
It is innervated by the muscular collateral branches of the intercostal nerves and the subcostal nerve.
Subcostalis acts to depress the lower ribs, an action that is used in expiration.