The pectoralis major muscle is subject to various morphologies. One presumably very rare variation is insertion of this muscle into the shoulder joint. During the routine dissection of the right upper extremity of an adult male cadaver a distinct separate tendinous insertion into the shoulder joint capsule was identified. This vertical tendon (7 mm × 6 cm) traveled just lateral to the long head of the biceps brachii muscle and terminated into the fibrous aspect of the joint capsule without penetrating it. Mechanical traction on the humeral attachment of the pectoralis major resulted in anteroinferior displacement of the shoulder joint capsule. The possible embryologic origin of this variation is discussed. Clinicians may wish to consider potential attachment of the pectoralis major into the capsule of the shoulder joint in diagnosing pathology of this region. © The Author(s). Journal compilation © Royal Economic Society 2009.