Mordellistena y-nigrum is a species of beetle in the family Mordellidae. It was described in 1937 by Eugene Ray from Puerto Rico. It is named for its diagnostic characteristic, a Y-shaped black spot on the region of the elytra surrounding the scutellum. This beetle measures in length, or when including the anal stylus. The antennae are long. References y-nigrum Beetles of North America Insects of Puerto Rico Endemic fauna of Puerto Rico Beetles described in 1937