Four new species of Begonia are described from Northern, Peninsular and Southwestern Thailand. Three belong to Begonia sect. Diploclinium (B. exposita Phutthai & M.Hughes, B. pengchingii Phutthai & M.Hughes and B. pseudosubperfoliata Phutthai & M.Hughes), and one (B. phutthaii M.Hughes) to Begonia sect. Apterobegonia. All four species are karst limestone endemics. The provisional IUCN status of all the new species is ‘Vulnerable’.