In response to temporal changes in the quality and availability of food resources, consumers should adjust their foraging behavior in a manner that maximizes energy and nutrient intake and, when resources are limiting, minimizes dietary overlap with other consumers. Floodplains of the Amazon and its lowland tributaries are characterized by strong, yet predictable, hydrological seasonality, seasonal availability of fruits, seeds, and other food resources of terrestrial origin, and diverse assemblages of frugivorous fishes, including morphologically similar species of several characiform families. Here, we investigated how diets of frugivorous fishes in the Amazon change in response to fluctuations in food availability, and how this influences patterns of interspecific dietary overlap. We tested predictions from classical theories of foraging and resource competition by estimating changes in diet breadth and overlap across seasons. We monitored fruiting phenology to assess food availability, and surveyed local fish populations during three hydrological seasons in an oligotrophic river and an adjacent oxbow lake in the Colombian Amazon. We analyzed stomach contents and stable isotope data to evaluate temporal and interspecific relationships for dietary composition, breadth, and overlap. Diets of six species of characiform fishes representing three genera changed according to seasonal fluctuations in food availability, and patterns of diet breadth and interspecific overlap during the peak flood pulse were consistent with predictions of optimal foraging theory. During times of high fruit abundance, fishes consumed items to which their functional morphological traits seemed best adapted, potentially enhancing net energy and nutritional gains. As the annual flood pulse subsided and availability of forest food resources in aquatic habitats changed, there was not a consistent pattern of diet breadth expansion or compression. Nonetheless, shifts in both diet composition and stable isotope ratios of consumer tissues during this period resulted in trophic niche segregation in a pattern consistent with competition theory.