Deciduous Beech (Fagus spp.) forests have a highly disjunct distribution in the Northern Hemisphere, with greatest diversity in eastern Asia. Although Fagus supports a rich phytophagous insect fauna, the diversity, composition, and host specialization of insects associated with these trees in Asia are less well known compared with Europe and North America. For instance, two species of Catocala moths, both considered endemic to Japan, were previously known as the only beech-feeding specialists of this genus. However, two Catocala species were recently discovered from Taiwan during a survey of phytophagous insects on Taiwan Beech Fagus hayatae, a threatened plant. Identification of these two species of moths was determined by mtDNA (COI barcodes), adult morphology, including wing pattern and genitalia, and biology. Based on these multiple sources of evidence, we conclude that one species represents an undescribed species and the other a new subspecies of a species previously known only from mainland China. The taxonomic treatments in the present work include Catocala seiohbo sanctocula Hsu & Huang, subsp. nov., C. nimbosa Hsu & Wang, sp. nov. and C. nubila ohshimai, stat. nov. In Taiwan, Catocala spp. occur mainly in large, undisturbed patches of Taiwan Beech forest and are codependent on F. hayatae. Thus, they are not only considered to be at risk of co-extinction, but may serve as potential indicator species to monitor the extent and health of this threatened ecological community.