Species-unique genes that encode specific proteins and have no homologs in other species play certain roles in the evolution of species and adaptations to external environments. Nevertheless, the biological roles of unique genes in plant pathogenic fungi remain largely unknown. Here, four Magnaporthe-unique genes (MUG1–MUG4), which were highly expressed during the early infection stages, were functionally characterized in the rice blast fungus Magnaporthe oryzae. Subcellular localization assays revealed that Mug1, Mug2, and Mug4 were localized to the cytoplasm and that Mug3 was localized into the nuclei. Furthermore, through gene knockout and phenotypic analysis, only MUG1 was found to be indispensable for fungal virulence and conidiation. Detailed microscopic analysis revealed that the deletion mutants of MUG1 clearly exhibited reduced appressorial turgor pressure and invasive hyphal development. Taken together, our findings indicate that the Magnaporthe-unique gene MUG1 plays a vital role in infection-related morphogenesis and virulence in rice blast fungi and suggest the specific and important roles of species-unique genes.