Surface-initiated photoiniferter-mediated polymerization (SI-PIMP) offers great promise for many applications primarily due to it lacking the requirement of a toxic catalyst as well as not being limited to specific types of monomers. In this work, zwitterionic carboxybetaine (pCB) polymer thin films with uniform and controlled thicknesses at high surface packing densities were achieved via SI-PIMP. It was found that the polymerization proceeded linearly with both UV-irradiation time and monomer concentration. Subsequent studies using a surface plasmon resonance biosensor showed pCB films to be ultralow fouling to undiluted human blood plasma. Antibody immobilization combined with excellent postfunctionalized protein resistance enabled the detection of an antigen down to 1 ng/mL from undiluted human plasma. These results demonstrate SI-PIMP as an attractive alternative to other commonly used polymerization techniques for zwitterionic monomers, such as atom transfer radical polymerization.