Entrapment axillary neuropathy at the quadrilateral space, also known as quadrilateral space syndrome (QSS), is a rare cause of posterior shoulder pain. We present a case of an axillary schwannoma causing QSS evaluated by both conventional ultrasound and sonoelastography. The application of sonoelastography is useful in elaborating the elastic feature of a soft tissue tumor, which provides additional information to conventional ultrasound regarding its growth pattern, and can help the differential diagnosis and guide the management.