An unusual anatomic variation of the deltoid muscle was found in a 45-year-old female cadaver during dissection of the right upper extremity. The posterior fibers of the right deltoid muscle were enclosed in a distinct fascial sheet and the deltoid muscle was seen to arise from the middle 1/3 of the medial border of the scapula. There was no accompanying vascular or neural anomaly of the deltoid muscle. To the best of our knowledge, unilateral posterior separation of the deltoid muscle with a distinct fascia has not been described previously. While dissecting deltoid, posterior deltoid, or scapular flaps, the surgeon needs to look out for this variation because it may cause confusion.