Trophic ecology is essential for understanding ecosystems and species interactions. Phylogenetically close groups can coexist in the same habitat and feed on similar items, resulting in niche overlap, which can compromise individual survival. In the Amazon, variations in river levels due to floods and droughts directly affect access to food resources for fish fauna. The objective of this study was to assess the influence of river discharge on the composition and dietary niche overlap of sympatric species Geophagus argyrostictus and G. altifrons in the Volta Grande of the Xingu River (Pará – Brazil). Monthly collections were conducted over a twelve-month period, and the specimens' diets were analyzed in a laboratory, with food items identified and weighed. The data were subjected to the Food Importance Index (IAi%) and Pianka's Overlap Index. The diet of G. argyrostictus consisted mainly of sediment (90.6%) and terrestrial plants (6.3%); G. altifrons fed on sediment (60.7%), terrestrial plants (31.4%), and aquatic insects (6.5%). We observed variations in diet composition between dry and wet seasons (PERMANOVA; p = 0.008) and between flood and recession periods (p = 0.037) for G. argyrostictus, as well as variations between flood and dry seasons (p = 0.004), flood and recession periods (p = 0.001), and recession and dry seasons (p = 0.001) for G. altifrons. Regarding niche overlap, Pianka's index values ranged from 0.39 during floods to 0.99 during dry seasons, with a significant effect of river discharge on niche overlap (GLM; t = -2.234, p = 0.049), which was higher during low-water periods compared to high-water periods. Our findings highlight the influence of river discharge on the diet composition of both species, which was more diverse during high-water periods. Also, we observed a greater dietary niche overlap during low-water periods, likely due to limited access to varied food resources, potentially resulting in interspecific competition.