Ion-doped poly(p-phenylenediamine) (Fe-ppd) nanoparticles were prepared at room temperature by using FeCl3 as an oxidant. Fe-ppd exhibited high photothermal conversion efficiency (39.27%) and excellent photostability. In vitro and in vivo evaluation demonstrated that Fe-ppd could be used as an ideal photothermal agent for photothermal therapy for the first time.