Several keystone herbivores in boreal forests feed on both freshwater and terrestrial plants. Quantifying the amount of freshwater and terrestrial plant biomass in the diets of these species is paramount to understanding their dietary ecology and role in mediating trophic linkages across these ecosystems. We examined the potential to distinguish freshwater and terrestrial plant sources with δ¹³C and δ¹⁵N stable isotope analysis. We collected freshwater and terrestrial vascular plants that are consumed by vertebrate herbivores from 2 distinct and geographically distant subarctic regions in Canada. We consistently observed more positive isotopic values in freshwater than terrestrial vascular plants ( 1.8 to 10.4‰ for δ¹³C and up to 3.2%o for δ¹⁵N) whether the aquatic plants grew in lakes or rivers. We could correctly classify 71 to 100% of plant isotope values into freshwater and terrestrial groups at a locality with cluster analyses of both δ¹³C and δ¹⁵N values. Submergent freshwater and terrestrial plants were the most isotopically differentiated, whereas emergent freshwater plants growing near shorelines contributed to some isotopic overlap between freshwater and terrestrial categories. Collectively, these results establish that site-specific δ¹³C and δ¹⁵N separation between freshwater and terrestrial vascular plants can be used to quantify their respective contribution to herbivore diets and document trophic linkages across freshwater-terrestrial interfaces.