Betula hainanensis J. Zeng, B.Q. Ren, J.Y. Zhu & Z.D. Chen (Betulaceae), a new species from Hainan Island, China, is described and illustrated. It is distributed in tropical, montane rain forests and evergreen broad-leaved forests at elevations above 700 m a.s.l., mostly as scattered individuals and occasionally as small populations. Betula hainanensis belongs to the section Betulaster and differs notably from the related species B. alnoides, B. luminifera and B. fujianensis in its morphology and phenology. A key is presented that distinguishes the species in the section Betulaster of Betula.