Lower limb strength is crucial in pitching, as force output correlates positively with pitching velocity. The countermovement jump (CMJ) is a common test for assessing lower limb strength, and previous research has hinted at its potential to predict performance in adult baseball players. However, further research is warranted concerning adolescent players. This study investigated the relationship between CMJ lower limb power and fastball velocity in adolescent baseball pitchers. Thirty-two adolescent male baseball pitchers from junior high school baseball teams executed three CMJs and threw five fastballs from a custom-made pitching mound. A Kistler force plate (2500 Hz) recorded ground reaction forces (GRFs) during the CMJ, and a pocket radar gun measured ball velocity. The Pearson correlation coefficient assessed the relationship between CMJ variables and ball speed. A stepwise-forward multiple regression model determined the contribution of CMJ variables to predicting fastball velocity in adolescent baseball pitchers. The CMJ variables (braking force, peak force, braking rate of force development, braking impulse, propulsive impulse, peak power, rate of power development, and leg stiffness) correlated positively with ball velocity. The regression analysis revealed that absolute braking force and body height explained 43.3% of the variance in velocity. In conclusion, adolescent pitchers with higher absolute braking force during the CMJ and greater body height are likely to achieve higher fastball velocity. Lower limb strength, assessed via CMJ tests, may aid in predicting adolescent pitchers' performance. These findings emphasize the importance of lower limb strength for pitching velocity and provide guidance to coaches for developing training programs to enhance adolescent players’ fastball velocity.