A new species of endemic, insular Cnemaspis is described from Hon Khoai Island, Ca Mau Province, Ngoc Hien District, 18 km off the southern tip of the Ca Mu Peninsula in southern Vietnam. Cnemaspis psychedelica sp. nov. is the most uniquely colored of all southeast Asian Cnemaspis in that both sexes and all age classes have bright orange forelimbs, forelegs, hands and feet; a bright orange tail; a dense, yellow reticulum on the neck overlying thick, black, longitudinal lines; and a blue-gray to light purple trunk bearing yellow transverse bars on bright-orange flanks. It also differs from all other species of Cnemaspis in aspects of size and squamation. Cnemaspis psychedelica is the third insular endemic species of Cnemaspis from Vietnam and brings the total number of species in Vietnam to six. Its occurrence on one of the 92 islands in Rach Gia Bay underscores the necessity for continued surveys of these poorly known islands that are beginning to show a surprising degree of endemism and diversity.