Cancer of unknown primary (CUP) is a metastatic tumor for which a standardized diagnostic workup fails to identify the site of origin at the time of making the diagnosis. The most frequent primary origin site is the lung and pancreas (40%), and the second most frequent primary origin site is the gastrointestinal system. For male patients with CUP, serum prostate specific antigen (PSA) level and free PSA/total PSA ratio should be determined; if necessary, immunohistochemical staining for PSA and P504S should be performed from the metastatic mass. Young patients with CUP should be attentively evaluated to establish the diagnosis and start treatment. We report the case of a young man who presented with a giant shoulder mass of unknown primary site and was diagnosed as having prostate adenocarcinoma.