Although glucosamine is commonly consumed by athletes, its effectiveness in sports injuries is still under debate. We aimed to investigate the effects of glucosamine to the rehabilitation outcomes of anterior cruciate ligament (ACL) reconstructed athletes. Glucosamine-sulfate (1000 mg daily, for 8 weeks) was administered to half of the cohort of 30 male athletes, the other half used a placebo. Both groups received the same rehabilitation protocol. Knee pain and functions were evaluated by a visual analogue scale (VAS), International Knee Documentation Committee (IKDC) and Lysholm scores before and after oral administration. Additionally, an isokinetic test was performed after the administration period. The scores revealed significant improvements in both groups after 8 weeks, but no significant difference was detected between groups in any of the parameters. Glucosamine supplementation did not improve the rehabilitation outcomes of athletes after ACL reconstruction. This is the first study investigating this topic. Further studies will help to obtain clear evidence about glucosamine efficacy on ACL injured or ACL reconstructed athletes.