A 56-year-old woman on insulin therapy for diabetes visited our clinic due to whitish discoloration on the right index finger. Despite topical application of 1% lanoconazole solution, the lesion grew, causing paronychia. Direct microscopy revealed non-dermatophyte molds. Based on the morphological features and genetic analysis of the isolate, the pathogen was identified as Aspergillus flavus. The patient was diagnosed with proximal subungual onychomycosis due to A. flavus. Following itraconazole pulse therapy, she was cured in 6 months. To our knowledge, this is the first reported case of fingernail onychomycosis due to A. flavus in Japan.