Polyetheretherketone (PEEK) has the potential to replace traditional metallic materials as an orthopedic and dental implant because its elastic modulus is similar to natural bone, although its intrinsic bioinertness hampers its clinical application. In this study, sulfonate groups were introduced onto the PEEK surface through a single-step ultraviolet-initiated graft polymerization of vinylsulfonic acid sodium. Diffuse reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses confirmed the successful attachment of sulfonate groups to the PEEK surfaces. A decreased angle of contact suggested there was an increase in hydrophilicity as a result of endowing the PEEK surface with sulfonate groups. Atomic force microscopy revealed there to be no significant changes in surface topology and roughness as a result of the surface modification. In vitro cell adhesion, viability assay, alkaline phosphatase activity and extracellular matrix mineralization analyses, and real-time polymerase chain reaction analyses showed increased adhesion, spreading, and proliferation, and osteogenic differentiation capacity on the surface-sulfonated PEEK substrate compared to those on the unmodified PEEK. Thus, the results indicated that the surface-sulfonated PEEK has a great potential for use in orthopedic and dental implants.