Background: The coactivation (Co-A) of shoulder muscles that contribute to humeral head depression can lead to mechanical unloading of the subacromial structures during abduction and thus can be beneficial for patients with arthroscopic rotator cuff repair (ARCR). The present study aims to examine the effectiveness of humeral head depressor muscle Co-A (DM-Co-A) training on clinical outcomes in a sample of patients with ARCR. Hypothesis: We hypothesized that DM-Co-A training with medioinferior vector during glenohumeral exercises can improve clinical results in the rehabilitation of ARCR. Study Design: Randomized controlled single-blind study. Level of Evidence: Level 1B. Methods: A total of 27 patients who underwent ARCR after a medium-sized rotator cuff tear and completed their Phase 1 training with ≥80% compliance were included. Together with 14 weeks of conservative treatment (6 weeks of Phase 2 training and 8 weeks of Phase 3 training), synchronized “DM-Co-A Training” was applied to the treatment group with an electromyography (EMG) biofeedback (EMG-BF) device. Patients in the treatment group were asked to voluntarily activate the humeral head depressor muscles guided by visual and auditory feedback of the EMG-BF device during the Phase 2 and Phase 3 conservative treatment exercises performed by the control group. Demographic characteristics of the participants were recorded. Visual analog scale and universal goniometer were used to assess pre- and posttreatment pain severity and joint range of motion, respectively. The Disabilities of Arm, Shoulder and Hand Questionnaire, Revised Oxford Shoulder Score, Modified Constant-Murley Shoulder Score, and the Western-Ontario Rotator Cuff Index were used to assess functionality. Results: There was a clinically meaningful improvement in pain severity, active ROM excluding internal rotation, and functionality in the treatment group compared with the control group (P < 0.05). Conclusion: A 14-week duration DM-Co-A with EMG may be beneficial in the postoperative rehabilitation of patients after ARCR.