Photothermal therapy (PTT) has been recognized as a promising approach for cancer treatment due to its minimal invasiveness and low systemic side effects. However, developing a photothermal agent with accurate tumor imaging capability is a prerequisite for the efficient PTT. Here, we developed a series of ultra-pH-sensitive indocyanine green (ICG)-conjugated nanoparticles for fluorescence imaging-guided tumor PTT. These nanoparticles exhibited high fluorescence activation ratio (~100-fold) with sharp pH transition (ΔpH_on/off <0.25), and superior temperature response than free ICG. The in vivo imaging experiments demonstrated that the nanoparticles generated excellent tumor-to-normal tissue contrast through pH-triggered fluorescence activation in tumor sites, which provided information on tumor mass location, boundaries, and shape. Moreover, comparing to free ICG, the nanosystem had significantly longer blood circulation time and more accurate tumor targeting, providing efficient photothermal therapeutic effect against A549 tumor in living animals. In conclusion, this nanoplatform offers a potential strategy for imaging-guided cancer PTT.