A photothermally sensitive poly(N-isopropylacrylamide)/graphene oxide (PNIPAM/GO) nanocomposite hydrogel can be synthesized by in situ γ-irradiation-assisted polymerization of an aqueous solution of N-isopropylacrylamide monomer in the presence of graphene oxide (GO). The colors and phase-transition temperatures of the PNIPAM/GO hydrogels change with different GO doping levels. Due to the high optical absorbance of the GO, the nanocomposite hydrogel shows excellent photothermal properties, where its phase transitions can be controlled remotely by near-infrared (NIR) laser irradiation, and it is completely reversible via laser exposure or non-exposure. With a higher GO loading, the NIR-induced temperature of the nanocomposite hydrogel increases more quickly than with a lower doping level and the temperature can be tuned effectively by the irradiation time. The nanocomposite hydrogel with its excellent photothermal properties will have great applications in the biomedical field, especially as microfluidic devices; this has been demonstrated in our experiments by way of remote microvalves to control fluidic flow. Such an “easy” and “clean” synthetic procedure initiated by γ-irradiation can be extended for the efficient synthesis of other nanocomposite materials.