Radical-featured organic materials usually demonstrate significantly narrower bandgap than that of regular organic materials, which is beneficial for near-infrared (NIR) absorption and non-radiative decay process, making them good candidates for photothermal agents (PTAs) of photothermal therapy (PTT). However, most organic radicals are too short-lived to be separated and stable. Herein, a stable pyrylium-croconaine (CR) dye CR845 with high diradical character was synthesized and assembled into CR845 nanoparticles (NPs) to explore the photoacoustic imaging (PAI) and photothermal therapeutic performance under NIR irradiation in vitro and in vivo. CR845 exhibited a stable NIR absorption at 845 nm (ϵ=3.37×105 L mol-1 cm-1) and a relatively high photothermal conversion efficiency (PCE) of 58.8 %. When assembled into nanoparticles, CR845 NPs showed good photostability, good biocompatibility, favorable NIR photoacoustic potential, effective cancer cell killing and complete tumor elimination upon 850 nm laser irradiation. The work demonstrates a diradical-featured CR PTA with favorable PAI/PTT effects, which can help to provide new ideas for the development of NIR PTAs, and further promote the application of organic radical materials in the field of biomedicine.© 2024 Wiley-VCH GmbH.