This article reports a novel technology for photoinduced surface hydrogelation with regional precision. The technology is based on the photochemistry of dithiocarbamate and xanthate groups, both of which are dissociated into highly reactive radical pairs upon ultraviolet (UV) light irradiation. Photoreactive hydrophilic polymers containing dithiocarbamate or xanthate groups were prepared by radical copolymerization of N,N-dimethylacrylamide with vinylbenzyl N,N-diethyldithiocarbamate or 2-(ethylxanthate)ethyl methacrylate, respectively. Upon UV light irradiation, the films of the photoreactive copolymers were converted to water-absorbable cross-linked gels. Higher derivatization of the photoreactive groups and longer irradiation times resulted in higher gel yields and reduced swellability. X-ray photoelectron spectroscopic (XPS) analyses and water contact angle measurements showed that upon UV light irradiation of the photoreactive copolymer cast on a poly(ethylene terephthalate) (PET) film, hydrogels were simultaneously formed and fixed onto the PET film. When a mixture of the photoreactive copolymer with a bioactive substance such as heparin and urokinase was used, prolonged whole-blood-coagulation times for heparin-immobilized surfaces and fibrinolysis for urokinase-immobilized surfaces were observed. A micropatterned hydrogelated surface was prepared with regional precision under UV light irradiation through a photomask. The photochemical surface technology developed here is expected to be useful for conferring potent antithrombogenicity on biomedical devices.