Abstract: Medicinal plants used by ethnic groups are shaped by environmental constraints (local flora), vertical inheritance (ancestor‐descendant inheritance within an ethnic group), and horizontal transmission (cultural exchange between ethnic groups). Understanding these drivers is critical for deciphering traditional ethnic medicinal plant practices and human‐mediated plant dispersals. Here, we integrated a high‐resolution phylogeny of 5,194 medicinal angiosperms with ethnobotanical data from 36 Chinese ethnic groups. The results reveal that medicinal plant selection shows weak evolutionary constraints, and the inclusion of exotic species—those native to other regions of China but absent from a given ethnic group's area—does not alter the fundamental clustering patterns of ethnic medicinal floras. We identify a profound biogeographic divergence: Ethnic medicinal floras are split into distinct northwestern and southeastern clades, with the Hengduan Mountains region serving as a decisive natural barrier. While environmental factors drive this primary regional division, we also observe progressive convergence in medicinal plant use through horizontal transmission across both clades. This ongoing cultural homogenization poses a significant threat to the preservation of unique, localized ethnobotanical knowledge. Together, our findings provide a comprehensive framework for how environmental barriers and cultural exchange interact to shape the biogeographic structure of traditional medicine.