Context: Tennis playing generates specific adaptations, particularly at the dominant shoulder. It remains to be established whether shoulder-strength balance can be restored by sling-based training for adolescent recreational tennis players. Objective: To investigate the effects of a sling-based exercise for shoulder external rotators on external rotator muscle strength, internal rotator muscle strength, glenohumeral range of motion, and tennis serve performance. Design: Test-retest design. Setting: Tennis training sports facilities. Participants: Twelve adolescent male players volunteered to participate in this study (age: 13.3 ± 0.5 y; height: 1.64 ± 0.07 m, mass: 51.7 ± 5.8 kg, International Tennis Number: 8). Intervention: The procedure spanned 10 wk. For the first 5 wk, players performed their regular training (RT) twice a week. For the last 5 wk, a sling-based exercise (SE) for strengthening the shoulder external rotator muscles was added to their RT. Main Outcome Measures: Maximal isometric strength of shoulder external and internal rotator muscles and glenohumeral range of motion in external and internal rotation were assessed in both shoulders. Serve performance was also evaluated by accuracy and postimpact ball velocity using a radar gun. Results: No change was found in any measurement after the RT period. Significant increases in external (∼+5%; P < .001) and internal (∼+2%; P < .05) rotator muscle strength and in external/internal strength ratio (∼+4%; P < .001) were observed after the SE period. Serve velocity and accuracy were significantly improved after SE (∼+2% and ∼+24%, respectively; P < .05 for both), while no clinically meaningful alterations in range of motion were observed. Conclusions: Prophylactic intervention through SE for strengthening shoulder external rotator muscles appears effective in restoring strength balance at the dominant shoulder and may prevent adolescent tennis players from sustaining degenerative shoulder problems, which could later impair their performance of daily and work-related tasks.