In SE Asia two wild species of Cucumis L. occur, of which C. debilis W.J.de Wilde & Duyfjes, endemic to Vietnam, is new. The second species, C. hystrix Chakrav., is widely distributed but not common. Both species are described and illustrated. The genus is furthermore represented by the two well-known cultivated species C. melo L. (the melon) and C. sativus L. (the cucumber); both represented with a wild variety in the area. Cucumis debilis is characterized by a delicate habit, long male and female pedicels, respectively (15-)20 and c. 35 mm long, and by a glabrous, almost smooth ovary.