BACKGROUND: Risk factors for ulnar collateral ligament injury (UCLI) are unclear despite increasing injury rates. We sought to summarize UCLI risk factors in baseball players. METHODS: A computer-assisted search of 4 databases was performed using keywords related to UCLI risk factors. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines were used for study methodology. Odds ratios and 95% confidence intervals were calculated for dichotomous outcomes, and mean differences and 95% confidence intervals were calculated for continuous outcomes using a random-effects model. RESULTS: Thirteen studies qualified for inclusion. A greater nondominant (ND) shoulder internal rotation (IR) range of motion (ROM) at 90° abduction arm demonstrated strong evidence as a significant risk factor for UCLI (P < .001) compared with a control group. Mean overall velocity (P < .001), fastball velocity (P < .001), changeup velocity (P = .03), and curveball velocity (P = .01), as well as fewer years of player experience (P < .001), less humeral retrotorsion in the ND arm (P < .001), and greater absolute side-to-side differences in retrotorsion (P = .006) were all moderate-evidence risk factors compared with control groups. Strong evidence suggests total ROM arc at 90° abduction in the dominant arm was not a risk factor for UCLI (P = .81). CONCLUSIONS: Greater ND shoulder IR ROM and less humeral retrotorsion (in professional and amateur players) as well as pitching velocity (in professional players) demonstrated strong to moderate evidence as risk factors for UCLI. Dominant arm total arc of motion, external, or IR ROM were not risk factors for UCLI. Standardized collection and reporting of risk factors is recommended to more clearly elucidate definitive risk factors for UCLI.