The genus Lasianthus in Thailand was studied on the basis of herbarium specimens and field investigation in Thai natural forests and protected areas during 2016–2018. A total of 68 species, including two subspecies and two varieties, are recognized, and their synonymy, distribution, and ecology are provided. Two new species, Lasianthus puffii and L. sithammaratensis are here described as new to science based on morphological characters of fertile and vegetative organs. Lasianthus puffii is similar to L. cyanocarpus but differs from that species by being densely hirsute, with 6‒8 mm long hairs on branches, branchlets, leaf blade, petioles, and calyx (not tomentose with 2‒3 mm long hairs) and drupes with five pyrenes (not. four pyrenes). Lasianthus sithammaratensis resembles L. appressus but differs in having calyx cupular (not campanulate) and drupes with 4‒5 pyrenes (not eight). Among the Thai Lasianthus, four names are reduced to synonymy, one species was transferred to the genus Paralasianthus and excluded from Thai Lasianthus. Here, a key for the identification to all Thai Lasianthus species, as well as their nomenclature, lists of specimens examined, and notes on distribution and affinities, are provided.
