AbstractGanoderma mushrooms have been used to treat rheumatoid arthritis (RA) in East Asia. Whether Ganoderic acid A (GAA), the natural product extracted from Ganoderma, could be utilized to alleviate osteoarthritis (OA) is investigated in this study. Destabilization of the medial meniscus (DMM) model was constructed to reveal the in vivo effect of GAA. We found that GAA could significantly alleviate the pathology of DMM, as confirmed by the diminished maximum histologic scores. On the contrary, GAA could down‐regulate the relative expression of osteoprotegerin (OPG) and up‐regulate the relative expression of nuclear factor kappa‐B ligand (RANKL) in DMM cartilage and human articular chondrocytes (HC‐A) cells with diminished matrix metallopeptidase 13 (MMP‐13) secretion in the synovial fluid. It was further demonstrated that the serum concentration of OPG was correlated with the severity of osteoarthritis. All these data reveal that GAA could improve OA by regulating the RANKL/OPG ratio to inhibit the secretion of MMP‐13.