The assembly of gold nanoparticles (AuNPs) to AuNP assemblies is of interest for cancer therapy and imaging. Herein we introduce a new and general paradigm, thermally triggered AuNP assembly, for the development of novel intelligent platforms for cancer photothermal therapy (PTT) and multimodal imaging. Site-specific conjugation of a thermally sensitive elastin-like polypeptide (ELP) to AuNPs yields thermally sensitive ELP-AuNPs. Interestingly, ELP-AuNPs can in situ form AuNP assemblies composed of short necklace-like gold nanostructures at elevated temperatures and thus show strong near-infrared light absorption and high photothermal effect. These thermally responsive properties of ELP-AuNPs enable simultaneous photothermal/photoacoustic/X-ray computed tomographic imaging and PTT of melanoma after single intratumoral injection of ELP-AuNPs. The thermally triggered assembly of a variety of nanoparticles with optical, electronic, and magnetic properties into nanoparticle assemblies may open new ways for the establishment of intelligent platforms for various applications in biomedicine.