Objective: Irisin, a recently identified myokine, is implicated in protecting mice from obesity. This study was designed to examine the relation of irisin levels in serum and synovial fluid (SF) with the radiographic severity of osteoarthritis (OA). Methods: Our study included 215 patients with knee OA. Irisin levels in serum and SF were evaluated using an enzyme-linked immunosorbent assay. The progression of OA was assessed using Kellgren-Lawrence grading system. Results: Knee OA patients had lower serum irisin concentrations and increased serum C-reactive protein (CRP) levels compared with healthy controls. There were markedly decreased irisin levels in both the serum and the SF, as well as increased serum CRP levels of knee OA patients with Kellgren and Lawrence (KL) grade 4 compared with patients classified as KL grade 2 and 3. Furthermore, patients with KL grade 3 showed markedly reduced serum and SF levels of irisin, as well as increased serum CRP levels compared with patients classified as KL grade 2. Irisin levels in serum and SF of knee OA patients were negatively correlated with disease severity evaluated by KL grading criteria. Conclusion: Irisin levels in the serum and SF of knee OA patients were negatively correlated with disease severity evaluated by the radiographic KL grading criteria.