Postharvest fungal diseases cause substantial losses of fruits worldwide, creating an urgent demand for environmentally friendly control strategies with broad-spectrum antifungal activity. Plant-derived volatile organic compounds (VOCs) have emerged as promising alternatives to synthetic fungicides. In this study, the antifungal activity and mechanisms of octanal, a natural aldehyde from citrus essential oils, were investigated against Botrytis cinerea and Monilinia fructicola. Octanal fumigation significantly inhibited mycelial growth and conidial germination in a dose-dependent manner, effectively controlling gray mold in grapes, brown rot in nectarines, and reducing natural decay in strawberries without compromising fruit quality. Transcriptomic analysis revealed that octanal induced extensive transcriptional reprogramming, with downregulation of oxidative phosphorylation-related genes and upregulation of glutathione metabolism-associated genes, indicative of oxidative stress. Consistent with these findings, octanal triggered excessive intracellular reactive oxygen species (ROS) accumulation, disrupted membrane integrity, caused ultrastructural damage, and reduced fungal cell viability. Overall, octanal exhibits potent broad-spectrum antifungal activity primarily through ROS-mediated oxidative damage, highlighting its potential as a safe and sustainable bio-fumigant for postharvest disease management.