The Tumenjiang River is an international river linking China, the Democratic People's Republic of Korea, and Russia. However, there has been little research into the mitten crab in this river. In this study, mitten crabs were sampled in the Tumenjiang River and their morphological traits were compared with other populations of Chinese mitten crab () sampled in the Yellow River and Liaohe River and with the Japanese mitten crab () collected on the Japanese mainland. Thirty-two morphological traits were examined with three kinds of multivariate analysis and a genetic cluster analysis. A discriminant analysis identified the lowest accuracy of discrimination among the Tumenjiang River crabs (83.30%). A principal components analysis revealed that the 12 largest differences in morphological traits in the Tumenjiang River population were between . A traditional clustering analysis showed the smallest differences between the Tumenjiang River crabs and the Japanese crabs. The results of a genetic clustering analysis also revealed that 60% of the individual crabs in the Tumenjiang River clustered with E. sinensis, and the remaining 30% of individuals may have been an introgression type. These results indicate that the mitten crab population in the Tumenjiang River comprises overlapping distributions or a genetically mixed population of , and that its morphological traits are closer to those of .