Penicillium expansum is the most important postharvest pathogen causing decay in deciduous fruit during storage. GAL4 proteins are fungal-specific regulators that play crucial roles in the development and pathogenicity of fungal pathogens. Here, a GAL4-like protein (PeGlp) was identified in P. expansum. ΔPeGlp mutants showed impaired growth, heightened sensitivity to the osmotic, cell wall, and pH stresses, and reduced pathogenicity in fruit, accompanied by diminished tissue acidification and gluconic acid (GLA) accumulation. Further analysis showed that the PeGlp knockout mutant: i. showed reduced expression of global regulator LaeA, and patulin biosynthesis; ii. inhibited the expression of pH-responsive factor PacC and related regulatory genes; iii. mediated carbon utilization via inhibition of carbon catabolite repressor CreA and Leloir pathway-related genes. Our results indicate that PeGlp is a key regulator for P. expansum virulence by regulating the activity of several key factors as part of the capability to metabolize galactose during pathogenicity.