© 2020 The Author(s). Background: Baseball pitching injuries are increasing at an alarming rate. While weighted ball throwing programs may be effective at increasing pitching velocity, previous research has identified a 24% injury rate and a 3.3° increase in shoulder external rotation (ER) range of motion (ROM) after performing a 6-week program. However, previous research has not investigated, separately, the immediate effects of throwing underloaded and overloaded balls on ROM. The purpose of this study was to examine the acute effects of throwing differently weighted baseballs on shoulder ROM. By analyzing these differences, it may be possible to determine the specific weight range that may lead to the greatest increase in ROM and potential injury risk. Hypothesis: Throwing with weighted balls will result in an increase in shoulder ER ROM. Study Design: Randomized controlled trial. Level of Evidence: Level 2. Methods: A total of 16 male high school baseball pitchers agreed to participate in this study. The participants were (mean ± SD) 17.1 ± 1.0 years of age, 1.81 ± 0.09 m tall, and had a mass of 79.2 ± 11.1 kg. Each participant was tested on 3 different days, 1 week apart, with 3 different conditions in random order: (1) underload throwing, using regulation 5-oz baseballs and 4- and 2-oz balls; (2) overload throwing, using 5-, 6-, and 9-oz balls; and (3) extreme overload throwing, using 5-, 16-, and 32-oz balls. Each testing session began by measuring passive shoulder ROM (external rotation and internal rotation) using standard goniometric measurements. Participants then performed 3 throws with each weighted ball from 3 different positions (kneeling, rocker, and run-and-gun) for a total of 27 throws each test session. ROM measurements were repeated at the end of each test session. The effect of each throwing condition on ROM was compared from pre- to posttraining using a paired t test (P ≤ 0.05). Results: There was no significant difference in ER after throwing at underloaded weights. The overload condition showed a statistically significant increase of 3.3° in external rotation (P = 0.05). The extreme overload condition showed a statistically significant increase in ER of 8.4° (P < 0.001). There were no differences in internal rotation for any group. Conclusion: A significant increase in shoulder ER was observed immediately after throwing overload weighted balls. This effect increased as the weights of the balls increased. Clinical Relevance: Throwing with overload weighted baseballs causes an immediate increase in shoulder ER ROM. It is unknown why these changes occur; however, the results may explain both the increase in velocity and injury rates previously observed from throwing weighted balls. The current study results may be used to develop more scientifically validated weighted ball programs. Heavier balls should be used with caution, and ROM should be monitored during implementation of these programs.