The genus Plethadenia Urb. consists of two geographically vicarious species: P. cubensis in eastern Cuba, and P. granulata in northern Haiti and the southwestern part of the Dominican Republic. The first collection of Plethadenia was made in Hispaniola by Bertero in 1819 or 1820. The next indication of this genus came from Cuban material (1909, Britton no. 1993). The small shrubs grow on limestone in dry thorn-scrub vegetations, in coastal thickets, and in dry forests. Flowers and fruits are extremely small. Ripe fruits and seeds were unknown until 1998 when a fruiting plant was found by the author in the Dominican Republic. Plant habit, the dehiscence of the follicles, and the seed presentation are described. The seed is ejected by a catapult mechanism. The systematic position within the subfamily Rutoideae sensu Engler is briefly discussed. Both species are likely endangered.