While 84% of patients expect to return to activity within 1 year of ACL reconstruction (ACLR), as few as 24% of patients will return to their preinjury level of activity. By considering a patient's perceptions of their reengagement in activity after ACLR, clinicians and researchers may be better equipped to implement interventions that are patient-centered. To describe the validation of the ACL Reasons survey, a tool to aid clinicians and researchers in understanding patient perceptions of barriers to physical activity (PA) engagement following ACLR. Cross-sectional study design. The ACL Reasons survey was administered via Qualtrics to 78 patients 6-24 months post-primary, unilateral ACLR. Patients were categorized as active, more challenging, or less active based on their responses to the ACL Reasons. ACL Reasons survey development occurred via an iterative process of drafting and revising based upon feedback from a team of external expert reviewers. Tegner Activity level, Marx Activity score, the Knee Injury and Osteoarthritis Outcomes Score (KOOS), ACL-RSI score, and Tampa Scale of Kinesiophobia (TSK-11) score were compared between groups using ANOVA and Kruskal Wallis tests. Groups significantly differed based on Tegner activity level (p<.001), Marx activity score (p=.01), ACL-RSI score (p<.001), TSK-11 score (p<.001), KOOS pain score (p=.02), KOOS symptom score (p=.04), KOOS sports and recreation score (p<.001), and KOOS quality of life score (p<.001) with the less active group performing worst on each. Knee symptoms, fear of knee symptoms or movement, and fear of injury were the most common reasons for change in PA engagement. These results support the validity of the ACL Reasons survey as a tool to identify barriers to PA engagement following ACLR. This tool may help facilitate communication between patients with ACLR and their healthcare provider to enhance patient-centered care.