Development of optical nanotheranostics for the capability of photodynamic therapy (PDT) provides opportunities for advanced cancer therapy. However, most nanotheranostic systems fail to regulate their generation levels of reactive oxygen species (ROS) according to the disease microenvironment, which can potentially limit their therapeutic selectivity and increase the risk of damage to normal tissues. We herein report the development of hybrid semiconducting polymer nanoparticles (SPNs) with self-regulated near-infrared (NIR) photodynamic properties for optimized cancer therapy. The SPNs comprise a binary component nanostructure: a NIR-absorbing semiconducting polymer acts as the NIR fluorescent PDT agent, while nanoceria serves as the smart intraparticle regular to decrease and increase ROS generation at physiologically neutral and pathologically acidic environments, respectively. As compared with nondoped SPNs, the NIR fluorescence imaging ability of nanoceria-doped SPNs is similar due to the optically inactive nature of nanoceria; however, the self-regulated photodynamic properties of nanoceria-doped SPN not only result in dramatically reduced nonspecific damage to normal tissue under NIR laser irradiation but also lead to significantly enhanced photodynamic efficacy for cancer therapy in a murine mouse model. This study thus provides a simple yet effective hybrid approach to modulate the phototherapeutic performance of organic photosensitizers.