Abstract: Melatonin has potent antioxidant, analgesic, and antinociceptive properties. However, the effects of melatonin against oxidative stress-induced cytotoxicity and inflammatory mediators in human chondrocytes remain poorly understood. This study examined the effects and underlying mechanism of melatonin in hydrogen peroxide (H2O2)-stimulated human chondrocytes and rabbit osteoarthritis (OA) model. Melatonin markedly inhibited hydrogen peroxide (H2O2)-stimulated cytotoxicity, iNOS, and COX-2 protein and mRNA expression, as well as the downstream products, NO and PGE2. Incubation of cells with melatonin decreased H2O2-induced Sirtuin 1 (SIRT1) mRNA and protein expression. SIRT1 inhibition by sirtinol or Sirt1 siRNA reversed the effects of melatonin on H2O2-mediated induction of pro-inflammatory cytokines (NO, PGE2, TNF-α, IL-1β, and IL-8) and the expression of iNOS, COX-2, and cartilage destruction molecules. Melatonin blocked H2O2-induced phosphorylation of PI3K/Akt, p38, ERK, JNK, and MAPK, as well as activation of NF-κB, which was reversed by sirtinol and SIRT1 siRNA. In rabbit with OA, intra-articular injection of melatonin significantly reduced cartilage degradation, which was reversed by sirtinol. Taken together, this study shows that melatonin exerts cytoprotective and anti-inflammatory effects in an oxidative stress-stimulated chondrocyte model and rabbit OA model, and that the SIRT1 pathway is strongly involved in this effect.