Abstract
Summary The genus Ziziphus (Rhamnaceae) is revised for Borneo. 13 species are recognised using morphological evidence, including three new endemic species: Ziziphus cuspidata , Z. domatiata and Z. puberula . Borneo is therefore the island with the greatest known diversity of Ziziphus species. The area surrounding Mount Kinabalu is particularly diverse, with nine species occurring in Ranau. Two new varieties of Z. borneensis are also described here, Z. borneensis var. ranggam and Z. borneensis var. velutina , five new synonyms are established, including the placement of Z. elmeri as a synonym of Colubrina beccariana . A taxonomic treatment, including a preliminary IUCN conservation status assessment, is presented for each species and variety.