The pursuit of phototheranostic agents with near-infrared II (NIR-II) emission, high photothermal conversion efficiency (PCE) and the robust generation of reactive oxygen species (ROS) in the aggregated state is always in high demand but remains a big challenge. Herein, we report a simple strategy to endow molecules with NIR-II imaging and photothermal therapy (PTT)/photodynamic therapy (PDT) abilities by equipping NIR-II aggregation-induced emission luminogens (AIEgens) with the cationic trimethylammonium unit, named as TDTN+. The resultant TDTN+ species can self-assemble into nanoparticles, which exhibit a maximum emission at similar to 1052 nm, a high PCE (66.7%), type I and type II ROS generation and a mitochondria-targeting ability, simultaneously. The TDTN+ can realize brain imaging with bright fluorescence and an effective tumor killing effect. Overall, this work presents an innovative design strategy to develop multimodality phototheranostic agents.