BACKGROUND: Elevated serum and synovial fluid (SF) YKL-40 levels have been detected in knee osteoarthritis (OA) patients. The current study was focused on the correlation between YKL-40 levels in serum or SF and symptomatic severity in patients with knee osteoarthritis. METHODS: 144 patients with knee OA and 151 healthy individuals were recruited into this study. Symptomatic severity was determined using Western Ontario and McMaster Universities Arthritis Index (WOMAC) scores from OA patients. Serum and SF levels of YKL-40 were explored by enzyme-linked immunosorbent assay. RESULTS: We found that YKL-40 levels in SF but not serum were independently and positively related to WOMAC pain (r = 0.531, p = 0.001), physical disability (r = 0.380, p = 0.025), and total scores (r = 0.407, p = 0.01) in knee OA patients. CONCLUSIONS: YKL-40 in SF could represent a potential biomarker for assessing the symptomatic severity of OA.