The genus Paris is an important and confusing taxon due to high variation within species, and differences between species are sometimes difficult to delimit. Thus, the status of some taxa has changed over time. To clarify the status of Paris species for plant conservation and effective management of this genus in Thailand, we performed an intensive survey in northern Thailand, studied morphological characteristics, and constructed a molecular phylogenic tree, which we compared to recently published results of this genus. Our results indicate that there are two species in Thailand: P. yunnanensis and a new species, P. siamensis. Detailed descriptions, illustrations, and the phylogenetic position of these two species are provided here.
Keywords: Melanthiaceae; Paris chinensis; Paris polyphylla; cryptic taxa; threatened species.