Gray mold caused by Botrytis cinerea is a devastating postharvest disease, resulting in severe fruit decay and enormous economic loss. Volatile secondary metabolites produced by plants or microorganisms are potential resources for developing fungicide alternatives. Here, we explored the antifungal activity of 3-octanol, a safe fungal volatile secondary metabolite, against B. cinerea. 3-Octanol exhibited a dose-dependent inhibitory effect on the development of B. cinerea both in vitro and in vivo. 3-Octanol treatment reduced the cell viability and suppressed the conidial germination of B. cinerea. Moreover, the transmission electron microscope analysis suggested that 3-octanol induces autophagy and autolysosome formation of B. cinerea. Further quantitative real-time PCR analysis indicated that 3-octanol induces the expression of autophagy-related genes and inhibits the expression of germination-related genes. These results suggest that 3-octanol controls gray mold by inducing autophagy, reducing cell viability, and suppressing the conidial germination of B. cinerea. Our findings suggest the potential role of 3-octanol in managing gray mold on postharvest fruit and provide a theoretical basis for the antifungal activity of 3-octanol against B. cinerea.