A new species, Alstonia undulifolia Kochummen & Wong, is described from the Malay Peninsula. Two sections of the genus occur in the Malay Peninsula, Alstonia sect. Monuraspermum Mon. and Alstonia sect. Alstonia, the latter being the correct name for what was previously known as sect. Pala (Adr. Juss.) Benth. Various characteristics, including growth architecture, are examined for their usefulness in distinguishing these two sections of the genus. In comparing A. angustiloba Miq. and A. pneumatophora Berger, both of which have not been properly differentiated by characteristics of the reproductive organs, A. pneumatophora var. petiolata Mon. is reduced to synonymy under A. angustiloba. A key to the seven species of Alstonia native to the Malay Peninsula is provided.