Photodynamic therapy (PDT) and photothermal therapy (PTT) are considered to be one of the most effective methods for treating cancer due to their noninvasive nature, high effectiveness, and fewer side effects compared to standard therapeutic modalities for cancer. However, conventional always-on types of PDT and PTT agents have basic drawbacks in their in vivo applications, which include the unwanted generation of strong fluorescence signals and phototoxicity in normal tissues, including blood vessels, when exposed to light, resulting in poor imaging contrast and unwanted phototoxicity. Here, we propose indocyanine green-loaded quenched nanoliposomes (Q-ICG-NLs) as an activatable nanotheranostics. Q-ICG-NLs showed significant quenching in near-infrared fluorescence emission and singlet oxygen generation upon light irradiation. The photothermal effect of Q-ICG-NLs was 1.3 times greater than free indocyanine green. Its fluorescence and singlet oxygen generation were largely restored when taken up into cancer cells, enabling the selective detection and phototherapy of cancer cells. These results suggest that Q-ICG-NLs can be effectively used for selective near-infrared fluorescence imaging and the subsequent image-guided PDT and PTT of cancers.