Monolepta ongi , n. sp., is described from Lanyu Island, Taitung county, Taiwan. Its allied species, Monolepta longitarsoides Chûjô, 1938, is redescribed for comparison. Their antennae, male aedeagus, female spermatheca and bursa-sclerites are illustrated. The lectotype and paralectotypes are designated for Monolepta longitarsoides from Chûjô's type series.