Objective: In the present study, we aimed to detect the levels of IL-17 in the serum, synovial fluid, and synovial membrane of patients with knee osteoarthritis (KOA) and investigate their association with the severity of articular cartilage damage and synovitis. Methods: According to the American College of Rheumatology criteria for osteoarthritis (OA) diagnosis, 46 cases diagnosed with knee OA (KOA group) were selected from patients that underwent arthroscopic surgery at our hospital from June 2012 to June 2014. Additional 46 cases with other knee joint diseases (non-KOA group) were included as controls. The IL-17 levels in synovial fluid and serum were assayed by ELISA, and those in the synovial membrane were assayed by immunohistochemistry. The severity of knee articular cartilage damage and synovitis-related pathological changes was evaluated by arthroscopy using the Outerbridge and Ayral scores, respectively. Results: IL-17 levels in the synovial fluid and synovial membranes of the KOA group were significantly higher than in the non-KOA group (P < 0.05). The levels of IL-17 in the synovial fluid and synovial membranes were positively correlated with the serum level of high-sensitivity C-reactive protein (HS-CRP), the Outerbridge score, and the Ayral score in the KOA group. Conclusion: IL-17 levels in the synovial fluid and synovial membrane are increased in KOA patients and are positively correlated with KOA severity.