In the Dissection Team of the Second Chair of Anatomy at the School of Medicine of the University of Buenos Aires, Argentina, during the routine dissection of 78 cadavers (corresponding to 156 supraclavicular fossae),10% formalin fixed, we found the supraclavicularis proprius muscle over the lower part of the left supraclavicular fossa in an adult Caucasian male cadaver. We described this rare muscular anomaly, the likelihood of finding this muscle, and its participation in supraclavicular nerve entrapment syndrome.