Nyssa yunnanensis W. Q. Yin ex H. N. Qin & Phengklai (Nyssaceae) is a critically endangered range-restricted tree species known from only three small populations in a tropical forest area of southern Yunnan Province, southwestern China. Two types of individuals occur, one bearing staminate flowers and the other with morphologically perfect flowers that produce both pollen and fruit, suggesting an androdioecious breeding system. Field and laboratory studies conducted between 2004 and 2007 indicate, however, that N. yunnanensis is functionally dioecious: pollen from the morphologically perfect flowers is inaperturate and inviable, rendering the trees that bear these flowers functionally female. Field observations showed that the staminate flowers opened 10 to 15 days earlier than the protogynous female flowers but that flowering ceased at nearly the same time in both sexes. Thirty-six species of insects were observed and collected visiting the flowers of N. yunnanensis, four of which served as effective pollinators. Breeding system experiments demonstrated that the predominantly entomophilous pollination system is supplemented by anemophily and further indicated that N. yunnanensis is xenogamous and does not appear to exhibit parthenogenesis. The average sex ratio of individuals within the three populations was female-biased (0.57∶1), but the ratio among flowers was male-biased (2.56∶1) because flower production was higher in males. The 37 known trees of N. yunnanensis are likely the remnants of a once more widespread, abundant species that has been heavily impacted by human disturbance, a fate shared with many other threatened Southeast Asian taxa whose continued survival will require dedicated conservation efforts informed by a detailed understanding of population structure and reproductive biology.