Second anterior cruciate ligament rupture is a common and devastating injury among young women who return to sport after ACL reconstruction, but it is inadequately understood. The purpose of this study was to compare gait biomechanics and return‐to‐sport time frames in a matched cohort of young female athletes who, after primary ACLR, returned to sport without re‐injury or sustained a second ACL injury. Approximately 6 months after primary reconstruction, 14 young women (age 16 ± 2 years) involved in jumping, cutting, and pivoting sports underwent motion analysis testing after physical therapy and impairment resolution. Following objective return‐to‐sport clearance, seven athletes sustained a second ACL rupture within 20 months of surgery (13.4 ± 4.9 months). We matched them by age, sex, and sport‐level to seven athletes who returned to sports without re‐injury. Data were analyzed using a previously validated, EMG‐informed, patient‐specific musculoskeletal model. Compared to athletes without re‐injury, athletes who sustained a second ACL injury received surgery sooner (p = 0.023), had post‐operative impairments resolved earlier (p = 0.022), reached criterion‐based return‐to‐sport benchmarks earlier (p = 0.024), had higher body mass index (p = 0.039), and walked with lower peak knee flexor muscle forces bilaterally (p = 0.021). Athletes who sustained a second injury also tended to walk with larger (p = 0.089) and more symmetrical peak knee flexion angles and less co‐contraction, all indicative of a more normal gait pattern. Statement of Clinical Significance: Delayed return‐to‐sport clearance even in the absence of gait or clinical impairments following primary ACL reconstruction may be necessary to mitigate second ACL injury risk in young women. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1894–1901, 2017.