The objective of the study was to determine if psychological readiness for sport and knee self-efficacy assessed early (3 months) after anterior cruciate ligament reconstruction (ACLR) are predictive of self-reported functional outcomes, quadriceps strength, and knee mechanics while running at the time of return to sport training (6 months). Thirty athletes with unilateral ACLR completed the ACL Return to Sport after Injury (ACL-RSI) and Knee Self-Efficacy Scale (K-SES) 3 months after ACLR and completed self-reported functional outcomes, isometric quadriceps strength testing, and three-dimensional running gait analysis 6 months after ACLR. The 3-month ACL-RSI significantly correlated with the 6-month International Knee Documentation Committee (IKDC;​ r = 0.565, p = 0.001), Knee Injury and Osteoarthritis Outcome Score (KOOS) sport/recreational activities (KOOS_Sport;​ r = 0.548, p = 0.002) and quality of life (KOOS_QoL;​ r = 0.431, p = 0.017), and quadriceps strength (r = 0.528, p = 0.003). The 3-month K-SES significantly correlated with the 6-month IKDC (r = 0.528, p = 0.003), KOOS_Sport (r = 0.430, p = 0.018), KOOS_QoL (r = 0.411, p = 0.024), quadriceps strength (r = 0.465, p = 0.010), and knee flexion excursion (r = 0.472, p = 0.008). With multivariate modeling, both the ACL-RSI and K-SES were predictive of the IKDC (R² = 0.411;​ p = 0.001). Only the ACL-RSI was predictive of the KOOS_Sport (R² = 0.300;​ p = 0.002), KOOS_QoL (R² = 0.186;​ p = 0.017), and quadriceps strength (R² = 0.279;​ p = 0.003), whereas only the K-SES was predictive of knee flexion excursion (R² = 0.173;​ p = 0.022). Athletes with greater psychological readiness for sport and knee self-efficacy at 3 months demonstrated higher scores on self-reported functional outcomes, greater quadriceps strength, and greater knee flexion excursion at 6 months after ACLR. This study indicates that psychosocial measures may be important to include into early post-surgical evaluations to help guide and facilitate interventions to restore subjective and objective knee function.