An os acromiale (OA) arises from a fusion failure of the anterior acromial apophysis. This case report presents the successful management of a painful OA associated to rotator cuff impingement in a competitive swimmer, based on ultrasonographic diagnosis and conservative management. Rest from sport activity, oral anti-inflammatory drugs and previous attempt of treatment of shoulder pain were ineffective. After two months of conservative treatment consisting of avoidance of swimming, local anti-inflammatory, physical therapy with ice, strengthening exercises with elastic bands to strengthen the scapular stabilizing muscles, rotator cuff and lowering humeral head muscles, the patient was pain free and all specific clinical tests for impingement syndrome (Neer, Hawkins, Whipple and Yocum tests) were negative. Digital compression of the OA site was not painful, and the Jobe and Palm-up tests were negative. The athlete returned to swim continuing the rehabilitation exercises, and the successful results were maintained at one year follow up. An unstable and symptomatic OA can be easily diagnosed with ultrasound exam. Rehabilitation for rotator cuff tendinopathies or/and bursitis can be a valid alternative to surgery.