BACKGROUND: This study sought to determine whether range of motion (ROM) of the ankle and subtalar joint complex (STJ) is correlated with ankle injuries in National Basketball Association (NBA) G-league and collegiate basketball players to identify an at-risk population that may benefit from participation in an ankle injury prevention program. METHODS: This prospective cohort study encompassed 103 player-seasons (68 collegiate, 35 NBA G-League). Patient demographics, passive ankle and STJ range of motion measurements, anterior drawer, and talar tilt tests were collected at preseason physicals along with plain radiographs. Subtalar eversion and inversion measurements were added to assess the Combination Motion (CM) of the STJ and subtracted to calculate the Subtalar Difference (SD). We defined the ratio of CM to SD as Subtalar Mobility Index (SMI=CM/SD). RESULTS: Twenty-one ankle injuries occurred with 10 405 player exposures yielding an incidence of 2.11/1000 exposures, resulting in 113 days of missed playing time. No direct measures of ankle, subtalar, or combined motion were associated with risk of injury, rejecting our original hypothesis that increased STJ ROM would predispose to ankle injuries. However, we did find that athletes with CM >16 degrees in combination with either SD ( .025) or SMI >3.75 ( = .032) were nearly 3 times more likely to have an ankle injury (3.14 vs 2.97, respectively). CONCLUSION: Using the predictive subtalar mobility thresholds found in this study may help identify at-risk players that may benefit from targeted ankle injury prevention programs. LEVEL OF EVIDENCE: Level II, prospective cohort study.