Aim: We aimed to translate, cross-cultural adapt and validate the Marx activity rating scale (MARS) of the knee for Romanian patients with anterior cruciate ligament (ACL) injury. Method: The original English form was translated according to guidelines. We included patients with ACL injury undergoing reconstruction in two centers over 3 years. Subjects completed the translated MARS, International Knee Documentation Committee (IKDC) subjective knee form and EuroQol EQ5D. The examining physician completed the Tegner Lysholm scale as an objective evaluation. Re-testing was obtained after one month. We used Spearman`s correlation to evaluate construct validity and reproducibility, Cronbach’s alpha for internal consistency and intraclass correlation for test-retest reliability. Results: We collected valid forms from 99 patients (32.1 ± 8.8 years, 64.6% males) during the preoperative evaluation and 45 were re-tested. Significant, very good correlations were found between the MARS and Tegner Lysholm (Spearman’s r = 0.712, p < 0.0001) and IKDC (Spearman’s r = 0.801, p < 0.0001). Cronbach’s alpha was 0.893 at the initial completion and 0.799 at re-test. The intraclass correlation coefficient was 0.895. Conclusions: The Romanian-translated MARS is a valid, consistent and reliable physical activity outcome measure in patients with anterior cruciate ligament reconstruction.