In 2021 and 2022, we found leaf spot symptoms in East Asian hogweed (Heracleum moellendorffii) plants grown in Pyeongchang and Yeongwol in Korea during disease surveys. Disease incidence in the plant leaves at the investigated locations ranged from 2 to 50%. Five single-spore isolates of fungi were obtained from the diseased leaves and identified as Phoma sp. based on their morphological and cultural characteristics. Phylogenetic analysis of the isolates revealed that the isolates belonged to the genus Didymella (anamorph Phoma) but were not grouped into the same species as any species in the genus, suggesting that they are a new Didymella species. Among the five isolates, three isolates were tested for their pathogenicity to East Asian hogweed plants through artificial inoculation. Pathogenicity of the isolates was verified in the inoculation test. The leaf spot symptoms induced by the inoculation test were similar to those observed in the investigated fields. This is the first report of Didymella sp. causing leaf spot in East Asian hogweed.