The Central American river turtle (Dermatemys mawii) is one of the of the most critically endangered turtle species worldwide, as a result of habitat loss and heavy hunting for consumption. Research on D. mawii becomes more challenging as wild populations are disappearing, while basic ecological data are still required to inform conservation efforts. In this study, our aim was to analyze the diet of D. mawii based on sex, and compare the information provided by stomach flushing and fecal samples. We examined both types of samples from individuals captured in 2017 and 2018 in a lentic system in the Lacantun River Basin, Chiapas, Mexico. We identified 3 broad categories (aquatic macrophytes, riparian resources and invertebrates) and 12 different food items, of which four are new reports of vascular plants for the diet of this turtle. The principal items consumed by D. mawii were Pistia stratiotes (a free-floating macrophyte) and Ludwigia sp. (a riparian plant). A low index of relative importance (≤ 0.5) for invertebrates suggests that they may be ingested accidentally when feeding on plants. We did not detect significant sex differences in the relative contribution of food items between the two sampling methods; additionally, we found a large dietary overlap (Cλ ≥ 0.89) between sexes that indicates a similar use of resources in this lentic system. We found all 3 broad food categories in stomach flushing samples and only one (riparian resources) in fecal samples; however, we did not find significant differences in the number of food items. The use of both postingestion sampling methods provided a more comprehensive view of the diet of this herbivorous freshwater turtle.