Measurements of the natural radiocarbon content of methane (14C-CH4) dissolved in seawater and freshwater have been used to investigate sources and dynamics of methane. However, during investigations along the Atlantic, Pacific, and Arctic Ocean Margins of the United States, as well as in the North American Great Lakes, some samples revealed highly elevated 14C-CH4 values, as much as 4–5 times above contemporary atmospheric 14C-CH4 levels. Natural production of the 14CH4 isotopologue is too low to cause these observations nor can it explain the variations in location and depth. Numerous lab and field validation tests and blanks, as well as the relatively small number of samples that display these elevated values, all suggest that these signals are not derived from an unknown procedural issue. Here we suggest that the byproducts of nuclear power generation include localized discharges of the 14CH4 isotopologue into marine and aquatic environments, severely altering the measured 14C-CH4 isotopic signals. Since several of our sample sites are distant from on-land nuclear powerplants, we conduct further calculations concluding that the most elevated anomalies in 14C-CH4 likely originate with discharge from nuclear-powered vessels.