The aim of this cross-sectional study was to investigate isometric internal rotation (IR), external rotation (ER), abduction (ABD), and eccentric external rotation (eccER) shoulder strength and rotational range of motion (ROM) in adolescent male and female competitive tennis players. Additional aims of the study were to provide a tennis-specific normative database based on a large sample of players to deepen the knowledge regarding shoulder strength and ROM for adolescent competitive tennis players, and to discuss differences based on sex, age, and level of play. Shoulder strength and ROM was assessed in 301 adolescent competitive tennis players, 176 boys and 125 girls with a mean age of 14.6 and 14.4 years, respectively. Outcome variables of interest were isometric IR and ER strength, ABD strength, eccER shoulder strength, intermuscular strength ratios ER/IR and eccER/IR, IR ROM, ER ROM, and total range of motion (TROM). A General Linear Model two-way ANOVA was used to analyze differences in sex, age, and level of play. The findings of this study demonstrated age, side, and sex differences in the shoulder isometric strength, the eccER strength and ROM in adolescent competitive tennis players. Furthermore, when strength was expressed as ratios ER/IR and eccER/IR both sexes showed a lower ratio for eccER/IR in national players (0.95 ± 0.22 and 0.95 ± 0.23) compared to regional players (1.01 ± 0.32 and 1.07 ± 0.29) for male and female players, respectively. In conclusion, this paper presents a tennis-specific normative database for shoulder rotation strength and ROM in adolescent male and female competitive players. The key points in this evaluation are strength values normalized to body mass, intermuscular ratios, and TROM.