Penicillium expansum is an important postharvest pathogenic fungus of fruits and vegetables and is capable of producing the mycotoxin patulin (PAT), posing a serious threat to food safety. To clarify the role of the APSES family transcription factor Xbp1 in the growth, development, pathogenicity, and PAT biosynthesis of P. expansum, the PeXbp1 gene was identified through whole-genome sequence analysis. A PeXbp1 deletion mutant was constructed by homologous recombination, and its biological functions were characterized through phenotypic analysis, stress response assays, pathogenicity tests, Reverse Transcription Quantitative Polymerase Chain Reaction (RT-qPCR), and High-Performance Liquid Chromatography (HPLC). The results showed, compared with the wild-type (WT) strain, the colony diameter, biomass, and conidial production of the ΔPeXbp1 mutant decreased by approximately 5.93%, 16.67%, and 9.29%, respectively, after 7 d of cultivation. In addition, the spore germination rate of the mutant was approximately 23.47% lower than that of the WT strain at 12 h, while the expression levels of LaeA and WetA were decreased by 33.33% and 23.34%, respectively (P < 0.05). Under different chemical stress conditions, the ΔPeXbp1 mutant showed no significant differences in tolerance to osmotic stress (NaCl and KCl) or cell wall/membrane stress (CR and SDS) compared with the WT strain. The lesion diameters caused by the ΔPeXbp1 mutant on apple and pear fruits were 12.27% and 30.76% smaller, respectively, than those caused by the WT strain, at 7 d post-inoculation. Furthermore, PAT contents in infected apple and pear fruits showed reductions of 22.52% and 33.69%, respectively, and PAT accumulation in liquid culture decreased by 16.94% after 7 d of incubation. Meanwhile, the expression levels of the key PAT biosynthetic genes PatH and PatJ were down-regulated by 45.11% and 45.80%, respectively (P < 0.05). These results indicate that PeXbp1 plays an important regulatory role in the growth and development, asexual development, pathogenicity, and PAT biosynthesis of P. expansum, providing a theoretical basis for the development of environmentally friendly strategies to control P. expansum and mitigate PAT contamination.