Leptocereus demissus Areces, from the dry limestone terraces of Pedernales province, southwestern Dominican Republic, is newly described and compared with its local relative, the widespread L. weingartianus. The chiefly 5–6-ribbed new taxon is distinguished from the latter species in its consistent scandent-like habit, its slender and drooping stem segments with straight to slightly dentate ribs lacking swollen podaria in their growing apices, its striking flowers with obconical receptacle-tubes that are not constricted above and below the nectar chamber (vs. narrowly tubular flowers that are distinctly constricted), and its spinier fruits with larger, persistent areoles, each bearing to 25 stiff spines that grow as the fruit matures, overlapping by their tips the spines of adjacent areoles. Because the 4-ribbed holotype of L. weingartianus — only referred to as coming from Haiti- was destroyed in 1943, a neotype of this species is hereby designated.