The aim of the study is to compare the effects of exercise training plus neuromuscular electrical stimulation with exercise training alone on shoulder function, pain, range of motion, and muscle strength in patients with subacromial impingement syndrome. Design: Patients were randomly divided into groups of exercise training (n = 24) and exercise training + neuromuscular electrical stimulation (n = 24). Shoulder function was evaluated with the Disabilities of the Arm, Shoulder and Hand Questionnaire, pain level was assessed with a visual analog scale, range of motion was measured with a goniometer, and muscle strength was assessed with a handheld dynamometer baseline and at the end of treatment (week 8). Results: In both groups, shoulder function, range of motion, and muscle strength (except flexion muscle strength in the exercise training group) increased, while pain decreased ( P < 0.05). Compared with the exercise training group, visual analog scale-activity and visual analog scale-night decreased more, and external-rotation range of motion and whole muscle strength increased more in the exercise training + neuromuscular electrical stimulation group ( P < 0.05). On the other hand, the effect sizes were medium to large for both groups. Conclusions: The addition of neuromuscular electrical stimulation treatment to exercise training did not improve shoulder function, which is the primary outcome, more than exercise training alone, but increased muscle strength and range of motion (external-rotation only) and decreased pain (activity-night), which are the secondary outcomes.