The aim of this study was to investigate the activation of the Wnt/beta-catenin pathway in osteoarthritis and the role of low-density lipoprotein receptor-related protein 5 (LRP5) in human osteoarthritic chondrocytes. The influence of 1,25(OH)2D3 on the expression of the LRP5 gene in human chondrocytes was also assessed. Human cartilage was obtained from 11 patients with primary osteoarthritis (OA) undergoing total knee replacement surgery. Normal cartilage was obtained from five healthy individuals. Beta-catenin and LRP5 mRNA levels were investigated using real-time PCR and LRP5 protein expression using Western blot analysis. Furthermore, we evaluated the effect of 1,25(OH)2D3 on LRP5 mRNA expression levels in osteoarthritic chondrocytes. Blocking LRP5 expression was performed using small interfering RNA (siRNA) against LRP5, and subsequent MMP-13 mRNA and protein levels were evaluated by real-time PCR and Western blot analysis, respectively. We confirmed the activation of the Wnt/beta-catenin pathway in OA, as we observed significant up-regulation of beta-catenin mRNA expression in osteoarthritic chondrocytes. We also observed that LRP5 mRNA and protein expression were significantly up-regulated in osteoarthritic cartilage compared to normal cartilage, and LRP5 mRNA expression was further increased by vitamin D. Also, blocking LRP5 expression using siRNA against LRP5 resulted in a significant decrease in MMP-13 mRNA and protein expressions. Our findings suggest the catabolic role of LRP5 is mediated by the Wnt/beta-catenin pathway in human osteoarthritis.