Sexual dimorphism is reported for the first time for the springtail Willowsia jacobsoni (Börner), this is the fourth record of conspicuous sexual dimorphism in the Entomobryidae. Typical males can be separated from typical females of the same age by the absence of pigment on the mesothorax, less abundant pigment on the fourth abdominal segment, and smaller size. These differences allow the separation of the sexes well before maturity. Aforementioned sexual differences apply to over 80% of the adults; few show variations which are discussed briefly. The species is redescribed on the basis of Puerto Rican specimens that represent the first record of the genus Willowsia from the Neotropical Region. The six proposed varieties of W. jacobsoni are discussed and considered as mere forms, although in a few cases that are pointed out they may represent different species. Head seta S6 may be a macro- or a microchaeta or it may be a macrochaeta, on one side of the head and a microchaeta on the other side.