Stimuli-responsive anticancer agents are of particular interest in the field of cancer therapy. Nevertheless, so far stimuli-responsive photothermal agents have been explored with limited success for cancer photothermal therapy (PTT). In this work, as a proof-of-concept, a pH-responsive photothermal nanoconjugate for enhanced PTT efficacy, in which graphene oxide (GO) with broad NIR absorbance and effective photothermal conversion efficiency is selected as a typical model receptor of fluorescence resonance energy transfer (FRET), and grafted cyanine dye (e.g., Cypate) acts as the donor of near-infrared fluorescence (NIRF), is reported for the first time. The conjugate of Cypate-grafted GO exhibits different conformations in aqueous solutions at various pH, which can trigger pH-dependent FRET effect between GO and Cypate and thus induce pH-responsive photothermal effect of GO-Cypate. GO-Cypate exhibits severe cell damage owing to the enhanced photothermal effect in lysosomes, and thus generate synergistic PTT efficacy with tumor ablation upon photoirradiation after a single-dose intravenous injection. The photothermal nanoconjugate with broad NIR absorbance as the effective receptor of FRET can smartly convert emitted NIRF energy from donor cyanine dye into additional photothermal effect for improving PTT. These results suggest that the smart nanoconjugate can act as a promising stimuli-responsive photothermal nanoplatform for cancer therapy.