The Anterior Cruciate Ligament Return to Sport After Injury (ACL-RSI) scale was developed to measure an athlete's psychological readiness to return to sport after anterior cruciate ligament (ACL) injury and reconstruction surgery. The scale is being used with increasing frequency in both research and clinical settings.To generate and validate a short version of the ACL-RSI scale.Cohort study (diagnosis); Level of evidence, 2.The ACL-RSI scale was administered to 535 patients who had undergone ACL reconstruction surgery. Reliability (Cronbach alpha) was determined and factor analysis of the full scale was undertaken along with a process of item selection and elimination. A second group of 250 patients participated in a predictive validation analysis. This group completed the ACL-RSI scale at 6 months and reported return-to-sport outcomes 12 months following ACL reconstruction surgery. The predictive validity of both scales (full and short versions) was assessed by use of receiver operating characteristic (ROC) curve statistics.The scale was found to have high internal consistency (Cronbach alpha, 0.96), which suggested that item redundancy was present. After an item selection process, the scale was reduced to a 6-item format. Cronbach alpha for the short version was 0.92, and factor analysis confirmed the presence of 1 factor accounting for 71% of the total variance. Scores for the short version were significantly different between patients who had and those who had not returned to sport. Six-month ACL-RSI scores for both the full and short versions had fair to good predictive ability for 12-month return-to-sport outcomes (full version: area under ROC curve, 0.77 [95% CI, 0.7-0.8]; short version: area under ROC curve, 0.75 [95% CI, 0.7-0.8]).A 6-item short version of the ACL-RSI scale was developed from a large cohort of patients undergoing ACL reconstruction. The short version appears to be as robust as the full version for discriminating between and predicting return-to-sport outcomes. The short version of the ACL-RSI may be of use in busy clinical settings to help identify athletes who may find return to sport challenging.