Gold nanoparticles (AuNPs) coated with a diblock copolymer composed of poly(ethylene oxide) (PEO) and poly-(N,N-dimethylaminoethyl methacrylate) (PEO-b-PDMAEMA) were prepared, while their reaction with 1,3-propane sultone gave rise to quaternized PEO-b-PDMAEMA chains on AuNPs. Using pyrene as a probe equilibrated with polymer-coated AuNP aqueous solutions, the fluorescence measurement results show that the diblock copolymers tethered to AuNP surfaces could undergo a thermally induced solubility change characterized by either a LCST (with PEO-b-PDMAEMA) or a UCST (with quaternized PEO-b-PDMAEMA). In both solutions, the use of a water-soluble PEO outer block helps preventing aggregation of AuNPs upon dehydration (decreased solubility) of either PDMAEMA chains at T> LCST or quaternized PDMAEMA chains at T < UCST. Moreover, the reversible solubility change of block copolymer chains on AuNPs could be induced not only by heating the bulk solution but also, to some extent, by illuminating the solution at wavelengths close to the surface plasmon resonance (SPR) band of AuNPs due to a photothermal effect. The method presented is an easy way to prepare polymer-coated AuNPs that can have a stable dispersion in aqueous solution while exhibiting either LCST or UCST thermosensitivity.