Since their introduction in 1985, coypus (Myocastor coypus) have spread widely throughout South Korea and are now considered an invasive species, with negative impacts on both agriculture and native biodiversity. Management of the species began in 2005, and related research has focused on factors influencing population control. Cold weather may cause significant population declines but the basis of that susceptibility has yet to be identified. Therefore, based on the analysis of 28 coypus trapped on Eulsuk Island in theNakdong River over a 12-month period, we sought to: (1) investigate coypu diet and body condition using the relationships between the δ¹³C and δ¹⁵N stable isotope values of coypu liver and hind-leg muscle tissues, mean temperature, and body condition index (log weight/log body length); (2) clarify the relative use of aquatic and terrestrial food plants, and (3) determine seasonal variations in coypu diet. Carbon and Nitrogen isotope values both differed seasonally and, in winter, between adults and juveniles. Carbon, but not Nitrogen, isotope values were influenced by temperature in the weeks before sampling. The δ¹⁵N values of liver tissues were influenced by sex and life stage at low temperatures; otherwise, with regard to diet, isotope ratios suggested that coypu primarily fed on aquatic vegetation. Coypus appear to make more use of the heavier nitrogen isotope in hind-leg muscle during winter, presumably associated with muscle tissue metabolism contributing to weight loss. During winter, these higher metabolic requirements together with the decreased availability of aquatic vegetation suggest that baiting near waterways in winter could be an effective method to control invasive coypu populations.