We describe reactive-monolayer-assisted thermal nanoimprint lithography. The reactive monolayer inducing the graft reaction with thermoplastic poly(styrene) by ultraviolet light exposure was formed from 4-((10-mercaptodecyl)oxy)benzophenone on a gold thin film. The photochemical graft reaction suppressed the thermally induced dewetting of a poly(styrene) thin film on the modified gold surface. As a result, the poly(styrene) thin film used as a resist layer for wet etching could be patterned by thermal nanoimprint lithography, and 100-nm-scale patterns of a gold thin film could be prepared simply by wet etching.