Abstract Phytochromes are red/far-red light receptors that regulate various aspects of plant growth, development, and stress responses. The precise mechanism by which phytochrome B (PhyB)-mediated light signaling influences plant defense and development remains unclear. In this study, we showed that PhyB enhances rice (Oryza sativa) blast disease resistance, tillering, and grain size compared to wild-type plants. Notably, PhyB interacted with and degraded grassy tiller 1 (GT1), a negative regulator of tiller development. Knockdown of GT1 in a phyB background partially rescued the diminished tillering of phyB. However, GT1 negatively regulates rice resistance to blast, suggesting that PhyB degradation of GT1 promotes tillering but not blast resistance. Previously, PhyB was found to interact with and degrade phytochrome-interacting factor 15 (PIL15), a key regulator of seed development that reduces rice resistance to blast and seed size. pil15 mutation in phyB mutants rescued phyB seed size and blast resistance, suggesting that PhyB might interact with and degrade PIL15 to negatively regulate blast resistance and seed size. PIL15 directly activated sugar will be eventually exported transporter 2a (SWEET2a). sweet2a mutants were less susceptible to blast disease compared to wild type. Collectively, these data demonstrate that PhyB promotes rice yield and blast resistance by inhibiting the transcription factors GT1 and PIL15 and downstream signaling.