The sea mink (Neovison macrodon (Prentiss, 1903): Mustelinae) was an unusual late-Cenozoic example of an organism that had rapidly evolved toward a marine niche. Except for the otters, it was probably the most aquatic member of the Musteloidea. Its status as a separate species has not been resolved. A larger relative of the American mink (N. vison), it inhabited the shores of New England and possibly the Canadian Maritime Provinces until it was hunted to extinction in the 19th century. Skeletal and skin specimens were not collected by zoologists, but the former are known from Native American archaeological sites. The hypothesis that the sea mink showed dental divergence from N. vison, an indication of systematic and ecological distinctness, was tested on 111 dentally mature mink specimens originally collected from the Turner Farm archaeological site (Penobscot Bay, Maine). These teeth, dating from about 5,000 to 250 years ago, were compared with 158 other specimens measured for this study and published data from 78 individuals, representing 4 subspecies of N. vison and 22 additional musteloid genera. Thirteen dental measurements were taken on all species and studied using regressions, principal component analysis, and significance testing. Based on comparisons with American mink, it appears likely that the archaeological specimens included primarily N. macrodon but also N. vison. Although pairs of species within the Lutrinae and genus Mustela showed divergence comparable to that of N. vison and N. macrodon, the dental proportions of male and female N. vison and of the several N. vison subspecies were nearly identical. These analyses suggest that N. macrodon was sufficiently distinct from N. vison to support its recognition as a separate species.