A new species Dillenia tetrapetala Joongku Lee, T.B. Tran & R.K. Choudhary (Dilleniaceae) is described from HonBa Nature Reserve of the Khanh Hoa province of Vietnam. Detailed illustrations and taxonomic comments are provided along with a table listing the differential characters to the closely similar taxa. Phylogenetic analyses using nrITS region of ribosomal DNA and psbA—trnH intergenic spacer region of chloroplast DNA sequences were also performed and they supported the status of D. tetrapetala as a distinct species. The plant is considered endangered based on the IUCN red list criteria because of its restricted distribution.