The interfacial properties of commercial poly(ether ether ketone) (PEEK) have been tailored by tethering polymeric brushes to the PEEK surface via surface-initiated atom transfer radical polymerization (SI-ATRP). The immobilization of an ATRP initiator on the PEEK surface was achieved by an unprecedented simple two-step wet chemical method. The keto groups at the PEEK surface were first wet chemically reduced to hydroxyl groups, and then 2-bromoisobutyryl groups were covalently anchored at the PEEK surface as ATRP initiator. SI-ATRP was performed at these functionalized PEEK surfaces with the three different monomers: potassium 3-(methacryloyloxy)propane-1-sulfonate (MPS), monomethoxy-terminated oligo(ethylene glycol)methacrylate (MeOEGMA), and N-isopropylacrylamide (NIPAAm). Atomic force microscopy, scanning electron microscopy, attenuated total reflection infrared spectroscopy, water contact angle measurements, and X-ray photoelectron spectroscopy ascertained the successful grafting of these polymer brushes at the PEEK surface. These brush-modified PEEK surfaces exhibited fully the physiochemical properties of the respective polymer brush: the surface with polyMPS brush showed selective staining by electrostatic interaction, while the polyMeOEGMA-modified surface was biorepellent. The surface modified with polyNIPAAm brush demonstrated a thermally responsive polarity change.