Current therapies against invasive pulmonary aspergillosis (IPA) have a limited cure rate. Given that a delay in treatment initiation may be fatal, a new drug with rapid -onset and potent antifungicidal activity is needed. The novel cyclic hexapeptide ASP2397 (currently known as VL-2397) exhibited antifungal activity against Aspergillus fumigatus (including azole-sensitive and azole-resistant isolates), A. terreus, and A. flavus at a MIC range of 1 to 4 μg/ml in human serum. Time-kill curve experiments showed that ASP2397 reduced germinated conidia of A. fumigatu by more than 1-log10 CFUs within 6 hours. In addition, ASP2397 inhibited hyphal elongation from germinated conidia of A. fumigatus, A. terreus, and A. flavus more rapidly than voriconazole. When treatment initiation was delayed in an IPA mouse model, ASP2397 had superior efficacy over posaconazole, with 100% survival and over 1-log10 CFU/g reduction in lung fungal burden. Histopathological investigation of lungs also showed that ASP2397 markedly suppressed disease progression. To clarify its mechanism of action, we generated a UV-induced mutant of A. fumigatus with low susceptibility to ASP2397. The mutant had a point mutation in the siderophore transporter gene sit1, which is absent in mammalian cells. These findings suggest that ASP2397 may improve clinical treatment options for IPA.