Mustela macrodon (extinct sea mink) is known only from prehistoric and historic Native American shell middens dating less than 5100 years old along coastal islands of the Gulf of Maine, northeastern North America. The species is distinct from all known extant subspecies of M. vison (American mink) but still belongs to the North American subgenus Vison. Metric comparisons between M. macrodon and five subspecies of M. vison, using skull, mandible, humerus, radius, femur, and tibia skeletal elements, show that M. macrodon is larger in overall size and robustness and is proportionately larger in the dental region. Many habitat-related parallels exist between coastal island mink of the Gulf of Maine and those of the Alexander Archipelago, southeastern Alaska, where the overall largest living subspecies of mink is found (M. v. nesolestes).