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 2.25mm in length, or 3.1mm when including the anal stylus. The antennae are 1mm long.