The single species within the genus Cassena occurring in Taiwan, C. sasajii Kimoto, 1969, is redescribed. Depositories of types of the species are reported. Diagnostic characters of both sexes are illustrated, including antennae, aedeagus, gonocoxae, spermatheca, ventrite V in males, and ventrite VIII in females. Host plants, distributional information, and overwintering behavior of adults are provided.