Msn2 is a Cys2His2 (C2H2) zinc finger transcription factor that plays crucial roles in oxidative stress responses in fungi. However, to date, the role of BcMsn2 in Botrytis cinerea, the causal agent of gray mold diseases in various plant species, remains uncharacterized. In this study, BcMsn2, an ortholog of Msn2, was identified in Botrytis cinerea. We found that BcMsn2 knockout mutant caused severely retarded vegetative growth, a hyperactive sporulation phenotype, and defects in the infection of host plants. To elucidate the molecular mechanism of the above phenotype, high throughput RNA-Seq showed that the expression of several genes involved in the balance of ROS was reduced in BcMsn2 knockout mutant, suggesting that BcMsn2 might function as a transcriptional regulator of these genes. These results indicate that BcMsn2 is involved in the vegetative differentiation, adaptation to oxidative stress, and virulence of B. cinerea.