A new species of Nyctaginaceae, Pisonia taina, is one of the rarest trees known from Puerto Rico. It has seldom been collected during the past four decades and had been, incorrectly, referred to several species names. It is readily distinguishable from the other five Pisonia of Puerto Rico by the venation on the under side of the leaf blade, which is raised through to the finest ramifications. The low number of individuals known and its confirmed existence in only two areas of Puerto Rico serve here to support its designation as a legally protected species.