The response regulator protein is a core element of two-component signaling pathway. In this study, we investigated functions of BRRG-1 of Botrytis cinerea, a gene that encodes a putative response regulator protein, which is homologous to Rrg-1 in Neurospora crassa. The BRRG-1 gene deletion mutant Delta Brrg1-62 was unable to produce conidia. The mutant showed increased sensitivity to osmotic stress mediated by NaCl and KCl, and to oxidative stress generated by H2O2. Additionally, the mutant was more sensitive to the fungicides iprodione, fludioxonil, and triadimefon than the parental strain. Western-blot analysis showed that the Bos-2 protein, the putative downstream component of Brrg-1, was not phosphorylated in the Delta Brrg1-62. Real-time polymerase chain reaction assays showed that expression of BOS-2 also decreased significantly in the mutant. All of the defects were restored by genetic complementation of the Delta Brrg1-62 with the wild-type BRRG-1 gene. Plant inoculation tests showed that the mutant did not show changes in pathogenicity on rapeseed leaves. These results indicated that Brrg-1 is involved in the regulation of asexual development, sensitivity to iprodione, fludioxonil, and triadimefon fungicides, and adaptation to osmotic and oxidative stresses in B. cinerea.