A novel near-infrared-responsive (NIR-responsive) photothermal therapy (PTT) agent based on perylene-diimide-encapsulated (PDI-encapsulated) PEGylated silica nanocapsules (SNCs) is developed. Dicyclohexylamino-PDI (DCAPDI) with electron-donating cyclohexylamino substitutes at bay positions aggregates into J-aggregation in the core of SNCs, and their electronic coupling interactions are strengthened because of the spatial confinement of SNCs, resulting in strong NIR absorption but negligible fluorescence emission which is crucial for NIR-responsive PTT. Based on our knowledge, this is the first example of generating NIR photothermal conversion by means of molecular aggregation derived from spatial confinement. Unprecedented photostability is achieved with the DCAPDI-encapsulated SNCs in response to more than 60 runs of cyclic NIR exposure with each run exposed to the 808 nm, 1 W cm -2 laser for 10 min. It overcomes the common photodegradation problem of small organic NIR dyes under continuous high-power laser irradiation thanks to the robust molecular skeleton of PDIs and their formation of structurally stable J-aggregates in SNCs. The DCAPDI-encapsulated SNCs demonstrate low cellular cytotoxicity and excellent in vivo photothermal efficacy in tumor ablation in a tumor-bearing zebrafish model, and thus allow the practical employment of a stable photothermal agent in clinical applications.