The marsh deer (Blastocerus dichotomus) is the largest deer native to South America, occurring in wetlands and marshy habitats. The southernmost population of the species is found in the Paraná River Delta, Argentina, in a wetland system highly modified by intensive forestry activity. Foresters perceive high levels of economic losses attributed to deer herbivory which drives marsh deer poaching. We carried out the first study of the dietary composition of the marsh deer in this wetland by using microhistological analysis of feces collected seasonally. Seventy-three food items were identified in the marsh deer diet, but only eight had frequencies of > 3% in the annual diet. Macrophytes dominated the diet throughout the year (seasonal percent frequencies 31-42%) due to the high occurrence of Ludwigia bonariensis (24% of average seasonal frequency). Trees, vines, forbs, and shrubs contributed less frequently to the diet, while grasses and grass-like plants were marginally represented (i.e., < 2%). Exotic plant species comprised 38% of the annual diet, and tree species of commercial importance (Salix sp. and Populus R22) were poorly represented (≤ 5.5% per season). The marsh deer in this wetland could be categorized as a browser, differing from the feeding behavior reported for the species in the Brazilian Pantanal (grazer-browser) and suggesting a trophic elasticity in this cervid. Since commercial tree species comprised a very small portion of the diet of this deer, a change in producers' perception toward the species is needed to diminish this conflict, and our data are important to develop context-specific conservation and management solutions.