Abstract: With many lakes, rivers, and marshes, the middle-lower Yangtze and Sanjiang plains are the two largest alluvial floodplains in China. After extensive surveys of the two areas, wild species of Trapa were found to be abundant in the two plains, and included 42 populations consisting of nine species (T. maximowiczii, T. incisa, T. quadrispinosa, T. bispinosa, T. japonica, T. manshurica, T. macropoda, T. octotuberculata, and T. mammillifer) and three variants (T. natans var . pumila, T. quadrispinosa var. yongxiuensis,and T. pseudoincisa var. nanchangensis). The morphological plasticity and taxonomic significance of the wild species of Trapa were evaluated using 16 classification characters. Results showed that: (1) the species and variants of Trapa were divided into two groups based on nut size and snout. Group A contained two species, T. maximowiczii and T. incisa; the remaining seven species and three variants were included in Group B; (2) for the five species and two variants with pseudohorns, cluster analysis of the morphological characters showed that T. maximowiczii and T. incisa were clustered together and separated from the other species and variants; and, (3) low taxonomic values were found for the characters of tubercles on nuts, leaves and petiole swellings. In conclusion, nut size and the morphology of the snout and pseudohorn offered the best diagnostic criteria for the classification of Trapa.