Background: Previous research has demonstrated that muscle synergy structure can adapt owing to training and injury; however, muscle synergies have not been evaluated in baseball players. Hypothesis: The throwing arm would have a similar muscle synergy structure but different levels of individual muscle activity within each synergy, relative to the nonthrowing arm. Study Design: Cross-sectional study in a controlled laboratory setting. Methods: Fourteen healthy competitive baseball players were included. Participants were tested bilaterally during a center-out planar reaching task using the KINARM robot, where kinematic data and surface electromyography data from 14 glenohumeral and scapular muscles were synchronized. Principal component analysis was used to extract muscle synergies, the variance accounted for (VAF) of each synergy, and individual muscle coefficients. The dominant (DOM) arm was compared with the nondominant (NDOM) arm using paired t tests for all dependent variables. Results: The same number of muscle synergies were extracted on the DOM and NDOM arms, along with no differences in VAF. In the first synergy, the infraspinatus (DOM 0.798 vs NDOM 0.587, P = 0.038) and lower trapezius (DOM 0.872 vs NDOM 0.480, P = 0.005) muscle coefficients significantly increased on the DOM arm. The second synergy had a significantly increased anterior deltoid (DOM 0.764 vs NDOM 0.374, P = 0.003) and a significantly decreased posterior deltoid (DOM −0.069 vs NDOM 0.197, P = 0.041) muscle coefficient on the DOM arm. Conclusion: The DOM shoulder exhibits a higher proportion of infraspinatus and lower trapezius muscle activation during the external rotation and abduction synergy. Also, the DOM shoulder has increased muscle activation of the teres major and latissimus dorsi during the internal rotation synergy, and increased muscle activation of the pectoralis major during the cross-body adduction synergy, compared with the NDOM shoulder. Clinical Relevance: By exploring these neuromuscular adaptations, the improved understanding of muscle synergy adaptations in baseball players will help optimize injury prevention and rehabilitation techniques. [ABSTRACT FROM AUTHOR]