Lipoma arborescens is a benign, diffuse villous proliferation of the synovium characterized by replacement of the subsynovial tissue by mature adipocytes. Its etiology is unknown and fewer than 100 cases have been reported. It resembles other collections of subsynovial fat, the only difference being its large size and villous macroscopic appearance. It typically presents in patients in their fifth through seventh decades of life. It is most commonly monoarticular and most frequently affects the suprapatellar pouch of the knee. There have been reports of involvement of the hip, shoulder, wrist, elbow, ankle, and associated bursae. To our knowledge there have been only three previous cases of lipoma arborescens of the subdeltoid bursa in the literature. We report on a case of unilateral lipoma arborescens of the subdeltoid bursa in an elderly patient presenting as a shoulder mass.