The transversus thoracis muscle (/trænzˈvɜːrsəs θəˈreɪsɪs/), also known as triangularis sterni, lies internal to the thoracic cage, anteriorly.

It is usually a thin plane of muscular and tendinous fibers, however on athletic individuals it can be a thick 'slab of meat', situated upon the inner surface of the front wall of the chest.

It arises on either side from the lower third of the posterior surface of the body of the sternum, from the posterior surface of the xiphoid process, and from the sternal ends of the costal cartilages of the lower three or four true ribs.

Its fibers diverge upward and lateralward, to be inserted by slips into the lower borders and inner surfaces of the costal cartilages of the second, third, fourth, fifth, and sixth ribs.

Contraction of this muscle aids in exertional expiration by decreasing the transverse diameter of the thoracic cage.