Objective: This study investigated the effects of an eight-week shoulder flexibility exercise program on shoulder range of motion (ROM) and serve accuracy in youth volleyball players. Methods: Thirty-two adolescent players (12–13 years) were randomly assigned to an experimental group (EG, n=16) and a control group (CG, n=16). The EG performed structured flexibility exercises twice weekly in addition to regular training, while the CG continued routine volleyball practice only. Shoulder ROM was assessed with a goniometer, and serve accuracy was evaluated using a standardized scoring protocol (maximum 48 points). Results: Significant improvements were observed over time in both groups for most ROM variables (p < 0.001). However, significant group differences emerged for shoulder external rotation (F_1,29=5.42, p=0.027, ηp²=0.153) and serve accuracy (Time × Group: F=6.73, p=0.015, ηp²=0.183). The adjusted ANCOVA confirmed a significant group effect for external rotation (F_1,29 =4.28, p=0.048, ηp²=0.128). The experimental group improved serve accuracy by +17.2 points (95% CI: 15.6–18.8) versus +11.6 points (95% CI: 9.4–13.8) in the control group. Conclusion: The eight-week flexibility program significantly enhanced serve accuracy and select shoulder ROM parameters in youth volleyball players. Incorporating structured flexibility exercises into training routines may improve technical performance and support shoulder function development during early adolescence.