Purpose: Repetitive and asymmetric movements in tennis can result in biomechanical adaptation in shoulder joint. The aim of this study was to investigate the differences in shoulder range of motion (ROM), strength, and functional performance tests between the dominant and nondominant shoulders, as well as to identify gender differences in junior tennis players. Methods: Forty-two junior tennis players (age mean: 11.3 [1.2] y, body mass index 18.3 [2.4] kg/m(2)) were included in the study. Shoulder internal rotation (IR), external rotation (ER) ROM, and total ROM, IR and ER isokinetic strength and closed kinetic chain upper-extremity stability, seated medicine ball throw used, grip hold tests were applied bilaterally. Paired sample t test and Student t test were used to compare the differences. Results: ER ROM was greater, while IR ROM and total ROM were lower on the dominant shoulder (all P values <.05). Nineteen players had glenohumeral IR deficit (IR ROM difference >13 degrees). The players had a greater ER strength on the dominant side and similar IR strength between shoulders. There was significant difference in seated medicine ball throw results between the dominant and nondominant sides (P < .001). The mean distance for bilateral seated medicine ball throw was 377.02 (85.70) m, and closed kinetic chain upper-extremity stability results were calculated as a mean of 15.85 (1.72) touches. Differences between the genders: total ROM of the dominant shoulder was higher in female players (P = .045), the IR PT/BW at 60 degrees/s angular speed was higher in male players' dominant shoulder (P = .030), and closed kinetic chain upper-extremity stability performance was higher in male players (P = .019). Conclusions: Adolescent tennis players demonstrated differences in strength, ROM, and functional performance results between the dominant and nondominant shoulders. Gender differences were also seen in the aforementioned parameters in junior tennis players. Determining these differences may improve our understanding of sport-specific shoulder joint adaptations in tennis.