Begonia hahiepiana H. Q. Nguyen & Tebbitt (Begoniaceae; section Sphenanthera (Hasskarl) Warburg), a new species from Phu Tho Province, Vietnam, is described and illustrated. This new species is most readily distinguished from the closely related B. balansana Gagnepain and B. ceratocarpa S. H. Huang & Y. M. Shui by its upper leaf surfaces, which are an iridescent dark blue (rather than glossy green) and have deeply sunken veins giving them a puckered appearance (rather than smooth), by its outer pair of tepals having a covering of red trichomes on their outer surfaces (rather than appearing glabrous throughout), and by its fruits, which are spherical with three thickened rib-like wings (rather than being either rhomboidal with three horned appendages or star shaped with six, or sometimes five or seven, fleshy wedge-shaped points). Fieldwork indicates that the new species has a restricted distribution and a total population of less than 1000 individuals; it is recommended to be placed in the IUCN category VU D 1, 2.