Begonia tui, a new species of sect. Coelocentrum, is described and illustrated here. It was collected during expeditions carried out on a limestone hill in Quang Binh Province, Central Vietnam. The new species is morphologically similar to B. mollissima in its rugose, widely ovate lamina. However, the new species can be distinguished by the margin of the stipule which is entire (vs. ciliate), androecium is zygomorphic (vs. actinomorphic), the pistillate flowers with 3 tepals (vs. 2), and the ovary's abaxial wing is triangular (vs. lunate). The conservation status of B. tui is assessed as Endangered, according to the IUCN Red List Categories and Criteria.