Background: The limited research on prognosis after nonsurgical management of anterior cruciate ligament (ACL) injury has focused on physical factors. We aimed to assess relationships between key patient-reported outcomes, in line with a biopsychosocial approach, and returning to preinjury sport at 12 months after ACL injury treated without reconstruction. Hypothesis: We hypothesized that biopsychosocial factors would be associated with returning to preinjury sport at 12 months after ACL injury. Study Design: Prospective single cohort study. Level of Evidence: Level 2. Methods: Patients who had an ACL injury and did not have reconstruction during the first year after injury were recruited from healthcare clinics in Sweden, and followed up at 3, 6, and 12 months after injury. Return to preinjury sport at 12 months was the primary outcome. Explanatory variables were psychological readiness to return to sport, knee-related quality of life, and self-reported knee function. Using generalized estimating equations, we evaluated the relationships between the explanatory variables and the primary outcome at each timepoint. Results: Data were analyzed for 88 participants with a median age of 27 years (15-40 years). Soccer was the most frequently reported preinjury sport (n = 22). Forty participants (46%) had returned to their preinjury sport at 12 months after ACL injury. The odds of returning to preinjury sport at 12 months increased with higher self-reported knee function at 6 months (odds ratio [OR], 1.1; 95% CI, 1.0-1.1), and the odds of being returned to the preinjury sport at 12 months doubled for every 1-point increase (1-10 scale) in psychological readiness to return to sport measured at 12 months (OR, 1.9; 95% CI, 1.2-3.2). Conclusion: Superior self-reported knee function at 6 months and greater psychological readiness to return to sport at 12 months were associated with returning to the preinjury sport 1 year after ACL injury treated without reconstruction.