A new structure, here named the “subantennal plaque sensillum,” found on the subantennal process of species of the cixiid genus Borysthenes Stål (Hemiptera: Fulgoromorpha: Cixiidae: Borystheninae), is reported. The new structure is a very fine pit area located on the dorsal surface at the anterior end of the subantennal process and is present in both male and female adults. Scanning electron microscopic examination of the pit area in Borysthenes maculata (Matsumura) showed that the pit area is a shallow cavity in which some cuticular infoldings are set. The cavity is about 32.0–34.5 μm in diameter and four subunits of the cuticular infoldings can be recognized in the center of the cavity. The structure is also present in the cixiid Euryphlepsia papuaensis Muir (Stenophlepsiini Metcalf 1938). This structure is externally similar to the plaque organs found on the surface of the antennal pedicel in species of some relatively advanced fulgoromorphan families, e.g., Issidae, Tropiduchidae (in part), Flatidae, Lophopidae, and Eurybrachidae. The function of the structure is not clear but it seems likely that it is a sensory structure. It is regarded as a synapomorphic character as it is not seen in any other cixiid taxa or other fulgoroid taxa as well. The presence of the new structure in all species of Borysthenes supports the monophyly of the genus. The occurrence of this structure in Euryphlepsia Muir also indicates that there may exist the close relationship between Borysthenes Stål (Borystheninae) and Euryphlepsia Muir (Stenophlepsiini).