Thermosensitive poly(N-isopropylacrylamide) (PNIPAM)/Au nanoparticle (NP) nanocomposite hydrogels are synthesized by in situ γ-radiation-assisted polymerization of N-isopropylacrylamide monomer aqueous solution in the presence of HAuCl4·4H2O. In this reaction, the PNIPAM hydrogels and the Au NPs are formed simultaneously, thus demonstrating an easy and straightforward synthetic strategy for the preparation of a uniform nanocomposite. The results suggest that increasing the monomer content during the preparation of nanocomposite materials can increase the sizes of Au NPs. The effects of irradiation dose and concentration of HAuCl4·4H2O on the optical and thermal properties of the hydrogel are also investigated. The PNIPAM/Au nanocomposite hydrogels act as an excellent catalyst for the conversion of o-nitroaniline to 1,2-benzenediamine, and the catalytic activity of the composite hydrogel can be tuned by the volume transition of PNIPAM. The in situ polymerization of monomer and reduction of metal ions initiated by a “clean” and “green” γ-radiation technique can be extended to the efficient synthesis of other nanocomposite materials.