An antifungal protein, AtUSP protein (At3g53990), was isolated from Arabidopsis thaliana leaves by ion and size chromatography and sequenced by N-terminal sequencing. The AtUSP gene amplified from an Arabidopsis leaf cDNA library was transformed to Escherichia coli to express the AtUSP protein. The recombinant protein inhibited the cell growth of various pathogenic fungal strains. The levels of the AtUSP transcripts were increased by various stresses, including pathogenic infection and salt stress. These results suggest that Arabidopsis AtUSP plays a critical role in the plant tolerance to diverse pathogenic infections. The potent antifungal action, which is a new function of AtUSP, was attributed to fungal reactive oxygen species (ROS) generation and mitochondrial potential alteration.