A biocompatible photothermal nanomedicine based on a PEGylated nanogel containing gold nanoparticles (GNPs) in a cross-linked network core of stimuli-responsive poly[2-(N,N-diethylamino)ethyl methacrylate] (PEAMA) gel for cancer photothermal therapy (PTT) was prepared through the reduction of Au(iii) ions without any reducing agents. The influence of the reduction conditions, such as pH, temperature, and N/Au ratio (molar ratio of the amino groups in the PEGylated nanogel to the Au(iii) ions), on the formation of the GNPs in the stimuli-responsive PEAMA gel core (reducing environment) was also studied. Note that the PEGylated nanogel containing GNPs prepared at pH 6, 60 degrees C and N/Au = 1 (PEGylated GNG (1)) was found to have the highest GNP-loading capacity with a diameter of about 8 nm, as observed by TEM; viz., about 27 GNPs formed in a single PEAMA gel core. PEGylated GNG (1) showed a remarkable photothermal efficacy (DeltaT = 7.7 degrees C) under irradiation with Ar ion (Ar(+)) laser (514.5 nm) at a fluence of 39 W cm(-2) for 6 min (14 kJ cm(-2)). Note that PEGylated GNG (1) showed non-cytotoxicity in the absence of irradiation with Ar(+) laser (480 microg mL(-1): > 90% cell viability), whereas pronounced cytotoxicity (IC(50) = 110 microg mL(-1)) was observed for PEGylated GNG (1) under irradiation with Ar(+) laser at a fluence of 26 W cm(-2) for 5 min (7.8 kJ cm(-2)), because of the heat-generation from the GNPs in the cells, which resulted in selective and noninvasive cancer PTT. Thus, PEGylated GNG (1), which has a high GNP-loading capacity, would be a promising nanomedicine for cancer PTT.