A detailed rheological characterization of synovial fluid from 22 patients undergoing total knee arthroplasty and three commercially available viscosupplements is performed. The results show that synovial fluid in osteoarthritis (OA) patients exhibits non-Newtonian shear thinning behavior and viscoelastic properties. Synovial fluid of the knees aspirated from one individual show very different viscosities and viscoelasticities. Moreover, rheopectic behavior is observed in OA synovial fluid at 37°C. All three viscosupplements exhibit non-Newtonian shear thinning behavior. The viscosupplement with cross-linked hyaluronic acid has a higher viscosity than that of with non-cross-linked hyaluronic acid. Moreover, high-molecular-weight viscosupplements have greater viscoelasticity than that of low-molecular-weight viscosupplements.