An unfused acromial epiphysis, called os acromiale, can become unstable and mobile when the deltoid contracts. This may cause pain and lead to impingement syndrome and rotator cuff tearing. After sustaining a direct blow to the right shoulder, a male division I basketball player was diagnosed with impingement syndrome and an os acromiale. Following failed conservative treatment, the athlete underwent arthroscopic subacromial decompression & debridement of the loose os acromiale in the right shoulder. One year later, following a fall on the left shoulder, the athlete was diagnosed with os acromiale, impingement syndrome and a superior labrum anterior-posterior (SLAP) lesion. Arthroscopic repair of the unstable type II SLAP lesion, together, with arthroscopic subacromial decompression, and resection of the os acromiale was performed on the left shoulder. Both surgeries were successful and the athlete was able to return to competition subsequent to completing a progressive shoulder rehabilitation program. Symptomatic os acromiale is rarely seen in young athletes. However, proper diagnosis and management is necessary for a successful recovery. Os acromiale should be considered as a part of the differential diagnosis in any athlete with rotator cuff impingement symptoms.