Background: Biomechanical efficiency, defined as fastball velocity per unit of normalized elbow varus torque, is a relatively new metric applied to improving the performance and health of baseball pitching. Purpose/Hypothesis: The purpose of this work was to evaluate kinematic parameters influencing biomechanical efficiency among professional and collegiate pitchers. Kinematic differences were compared between pitchers of high and low biomechanical efficiency. We hypothesized that professional pitchers would have greater biomechanical efficiency than collegiate pitchers. Study Design: Descriptive laboratory study. Methods: A deidentified biomechanical database of 545 pitchers (447 professional, 98 collegiate) was analyzed. A multivariate linear regression model was used to evaluate significant findings a priori with α =.05. Additionally, biomechanical differences were identified between competition levels and between high and low biomechanical efficiency groups using Mann-Whitney U test (α =.05). Results: Competition level and 11 (of 21) kinematic variables explained 27% of the variance in biomechanical efficiency, with most of the predictors being throwing arm kinematics (elbow flexion at stride foot contact [SFC]: β, −1.47; SE, 0.26; shoulder abduction at SFC: β, −1.78; SE, 0.39; shoulder external rotation at SFC: β, 0.60; SE, 0.22; maximum external rotation [MER] angle: β, 1.82; SE, 0.42; shoulder horizontal adduction at MER: β, −3.42; SE, 0.71) (all P≤.05). Professional pitchers had greater biomechanical efficiency than collegiate pitchers (711.0 ± 101.0 vs 657.0 ± 99.3, respectively; P <.001; d = 0.53). Compared with the low-efficiency group, the high-efficiency group had significantly lower normalized elbow varus torque with greater weight and height (high: 0.047 ± 0.004 %wt*ht vs. low: 0.063 ± 0.006 %wt*ht, P <.001; d = 3.20). At the instant of SFC, the high-efficiency group demonstrated greater shoulder external rotation and less elbow flexion, shoulder abduction, and pelvic rotation. The high-efficiency group also had greater MER and less shoulder horizontal adduction at MER, trunk side tilt at ball release, and knee excursion from foot contact to ball release. Conclusion: Professional pitchers had greater biomechanical efficiency than collegiate pitchers. Biomechanical efficiency was also affected by 11 kinematic variables identified in this study. Pitchers with higher efficiency had distinct differences in arm position, trunk side tilt, and lead-knee extension range of motion in the delivery. Thus, pitchers and baseball organizations should focus on these factors to lower normalized elbow varus torque relative to ball velocity.