Although ectopic insertion of the pectoralis minor has been previously described in the literature, the role of such anatomical variations in producing shoulder symptoms (i.e., pain and/or stiffness) remains controversial. We describe here a case of shoulder stiffness in which an ectopic insertion of the pectoralis minor over the supraspinatus tendon was found to be the main reason for severe contracture in external rotation and anterior thoracic pain. During arthroscopy, external rotation could not be restored despite anterior and inferior capsulotomy and release of the capsule of the rotator interval. Complete restoration of the passive external rotation was obtained only after complete release of the pectoralis minor from the supraspinatus. This observation underlines the importance of recognising such an anatomical variation when performing an arthroscopic arthrolysis in a stiff shoulder.