AbstractAchieving photothermal therapy (PTT) at ultralow laser power density is crucial for minimizing photo‐damage and allowing for higher maximum permissible skin exposure. However, this requires photothermal agents to possess not just superior photothermal conversion efficiency (PCE), but also exceptional near‐infrared (NIR) absorptivity. J‐aggregates, exhibit a significant redshift and narrower absorption peak with a higher extinction coefficient. Nevertheless, achieving predictable J‐aggregates through molecular design remains a challenge. In this study, we successfully induced desirable J‐aggregation (λ_abs^max: 968 nm, ϵ: 2.96×10⁵ M⁻¹ cm⁻¹, λ_em^max: 972 nm, Φ_FL: 6.2 %) by tuning electrostatic interactions between π‐conjugated molecular planes through manipulating molecular surface electrostatic potential of aromatic ring‐fused aza‐BODIPY dyes. Notably, by controlling the preparation method for encapsulating dyes into F‐127 polymer, we were able to selectively generate H‐/J‐aggregates, respectively. Furthermore, the J‐aggregates exhibited two controllable morphologies: nanospheres and nanowires. Importantly, the shortwave‐infrared J‐aggregated nanoparticles with impressive PCE of 72.9 % effectively destroyed cancer cells and mice‐tumors at an ultralow power density of 0.27 W cm⁻² (915 nm). This phototherapeutic nano‐platform, which generates predictable J‐aggregation behavior, and can controllably form J‐/H‐aggregates and selectable J‐aggregate morphology, is a valuable paradigm for developing photothermal agents for tumor‐treatment at ultralow laser power density.