The transversus thoracis muscle lies internal to the thoracic cage, anteriorly.

The transversus thoracis muscle is innervated by the intercostal nerve and superiorly attaches at the posterior surface of the lower sternum.

The function of this muscle is unknown, but it is part of the innermost intercostal muscle group along with the transversus thoracis muscle which is known to be a synergist in aiding the internal intercostal muscles with forced exhalation.

The posterior surface is concave, and directed backward and downward; that of the first gives attachment to the sternothyroideus, those of the third to the sixth inclusive to the transversus thoracis muscle, and the six or seven inferior ones to the transversus abdominis muscle and the diaphragm.