Abstract
Chayamaritia vietnamensis, a new species from Son La Province, northern Vietnam, is described and illustrated. The phylogenetic study revealed that the new species is most closely related to C. banksiae and C. smitinandii. The morphological comparison suggests it as the third new species of Chayamaritia and distinguished from C. banksiae and C. smitinandii by a combination of morphological characters of leaf blades, bracts, calyx and corolla, especially its peltate leaf blades. This species is provisionally assessed as endangered (EN B1ab(iii), B2ab(iii)) using IUCN Categories and Criteria. Information on ecology, phenology and an identification key for the known Chayamaritia species are also provided.