Although green light (GL, 500–600 nm) occupies half the visible light spectrum and regulates a series of plant developmental processes, the mechanism by which GL regulates seedling morphogenesis is enigmatic. Here, we reported that pure GL (500–600 nm, λmax, 527 nm) promoted the cotyledon development of Arabidopsis seedlings through phytochrome B (phyB). Genetic analysis indicated that phyB was involved in cotyledon development under continuous GL. Compared with red light (RL), GL induced phyB translocation from the cytoplasm to the nucleus and speckle-like photobody formation with a slower rate, which was reversed by far red light. Further transcriptomic data demonstrated that phyB participated in GL-responsive transcriptional networks in concert with four PHYTOCHROME-INTERACTING FACTORS (PIFs, PIF1, PIF3, PIF4, and PIF5). As expected, protein levels of the PIFs were decreased by GL. Similar to RL, GL induced the rapid degradation of PIF3 in a phy-dependent manner. Our results suggest that pure GL (527 nm) leads to the migration of phyB into the nucleus to trigger photomorphogenesis by possibly promoting the degradation of PIFs.