Anterior cruciate ligament (ACL) reconstruction is commonly performed to restore knee stability after ACL injury; however, graft healing remains a critical determinant of successful outcomes. This randomized, double-blind, placebo-controlled trial evaluated the efficacy and safety of adjunctive MSC-derived secretome injection administered to the graft during arthroscopic single-bundle ACL reconstruction using a peroneus longus tendon autograft. Twenty-four participants were randomized to receive either secretome injection or placebo. Functional outcomes were assessed using the IKDC Subjective Knee Evaluation Form and the Tegner-Lysholm Knee Scoring Scale before surgery and at six months postoperatively. The study evaluated whether adjunctive secretome injection could improve functional recovery compared with placebo while monitoring treatment safety.