The biogeographical history of several important vegetables is still unclear. In the gourd family, Cucurbitaceae, this applies to melon and cucumber, but also to many species of more regional importance. Cucumeropsis mannii is cultivated in West Tropical Africa for its nutritious seeds. Family-wide phylogenetic analyses suggested that it is closest to Posadaea sphaerocarpa from Central and South America, the seeds of which are also eaten and the fruit of which is made into bowls. To reconstruct these species' historical biogeography, we sequenced six plastid markers and the nuclear ribosomal ITS region for several accessions of both species, plus all relevant outgroups. Morphological traits were studied in 102 herbarium specimens representing both species. A 5,155 nucleotide-long matrix of chloroplast and nuclear DNA contained a single informative mutation in a poly-C region of nuclear ITS among six accessions that covered the species' native ranges. Next-closest species differed in all plastid markers and by ≥34 mutations in ITS1 and ITS2. Study of the morphology revealed a possible small difference in fruit shape (cylindrical-ovate versus spherical), presumably resulting from human selection on the African populations. The closest outgroups Melancium and Melothria are endemic to the neotropics, and maximum likelihood area reconstruction indicates that Cucumeropsis mannii also originated there. The near-absence of genetic and morphological differentiation implies that gene flow between Cucumeropsis manni and Posadaea sphaerocarpa stopped relatively recently, and taxonomically they should (or could) be treated as one species. Transport of seeds during the transatlantic slave trade is a possible scenario, although we cannot reject natural dispersal.