Leaf skeletonization represents a distinctive form of insect feeding behavior. It commonly occurs in angiosperm leaves after their initial appearance during the Early Cretaceous. This type of feeding behavior rarely has been documented in pre-Mesozoic fossils. We describe the earliest evidence of insect-skeletonized leaves of Dictyophyllum nathorstii Zeiller, affiliated with the extant fern family Dipteridaceae in the Late Triassic Yipinglang flora from southwestern China. The skeletonization generally is located adjacent to the pinna rachilla of the distal free portion of the leaf. In the skeletonized area, the interveinal tissue is completely removed, exposing the pinna rachilla, pinnule midveins, and lateral veins. Most nonvascular tissue has been removed between the vascular bundles, the latter forming polygonal meshes of varying size. Our report of insect-mediated skeletonization of fern leaves from southwestern China fills a spatiotemporal gap in the published data on the paleogeographical distribution and stratigraphic occurrence of plant–arthropod associations, and indicates an antagonistic relationship between a fern host and its insect herbivore.