Mallotus thinii Kim Thanh & V.S.Dang, a new species discovered in coastal areas of the south-central provinces of Vietnam, is described and illustrated here. This species is morphologically most similar to M. leptostachyus Hook.f. and M. peltatus (Geiseler) Müll.Arg. It can be distinguished from other Mallotus species previously known by its habitat on sandy soils close to the beach, shrubby habit, deciduous stipules, oblanceolate leaf blades, two larger extrafloral nectaries near the base or sometimes two to four more along the midrib, presence of a pistillode, and fruits without spines. A detailed description, color photographs, distribution, habitat, preliminary conservation assessment, and phylogenetic analysis are provided. Molecular phylogenetic analyses based on matK and ITS nucleotide sequences strongly confirm M. thinii as a distinct new species within Mallotus .