"Begonia hosensis", a new species of sect. "Reichenheimia" from Sarawak, Malaysia is here described and illustrated. "Begonia hosensis" resembles "B. andersonii", differing in the stipule prominently keeled and female flower 5-(vs. 4-) tepaled. Ecologically, the two species are also separable: "B. hosensis" occurs on granitic cliffs ca. 400 km east of Kuching, the only known locality of "B. andersonii", a limestone species.