Boron-dipyrromethene (BODIPY) dyes have aroused considerable interest in cancer theranostics over the past decade because of robust photochemical properties. Although a large number of BODIPY photosensitizers (PSs) containing heavy atoms have been reported, the development of heavy-atom-free BODIPY PSs for oncologic photodynamic therapy (PDT) has been limited mainly by the uncertainty in intersystem crossing (ISC) mechanisms, unbalanced phototoxic effectiveness and fluorescence quantum yields, and aggregation-caused quenching effects. In addition, the lack of extensive in vivo studies of heavy-atom-free BODIPY PSs continues to limit clinical application. Herein, novel heavy-atom-free BODIPY-phenoxazine triads (BDP-8/BDP-9) that generate efficient excited triplet states via radical pair intersystem crossing (RP-ISC) followed by triplet charge recombination were developed for use in fluorescence image-guided PDT. BDP-8/BDP-9 exhibited high molar absorption coefficients, prominent aggregation-induced emission, and excellent singlet oxygen generation capability upon light irradiation. The corresponding BODIPY nanoparticles (BDP-8/BDP-9 NPs) with bright red emission, considerable phototoxicity, and excellent tumor-targeting ability were simply prepared by encapsulating BDP-8/BDP-9 PSs in a polymeric matrix. More importantly, the results of both in vitro and in vivo experiments demonstrated the considerable potential of BDP-8 NPs for image-guided photodynamic cancer therapy. This study may inspire the development of potential BODIPY-based nanoagents for cancer theranostics.