The Guiana dolphin (Sotalia guianensis) is a small-bodied dolphin distributed along the Atlantic coast from Honduras to southern Brazil. It preys on fish, squid and shrimps. Several seminal studies have described its diet, yet relationships between the species’ feeding plasticity and climate gradients remain unknown. We compiled a large database of Guiana dolphin stomach remains from southeast coastal Brazil. We described the species’ diet using a number of descriptors, multivariate analysis of variance to test possible differentiation in diet composition, and the Morisita index to estimate the extent of trophic niche overlap between groups. We also analysed feeding plasticity using a regression tree analysis followed by an ordination analysis. We present new records of prey for the species in Brazil. Our results suggest that the Guiana dolphin has opportunistic feeding habits, which may exhibit the species’ feeding plasticity. Such feeding plasticity is associated with the capability to prey throughout a wide array of climate conditions. From a conservation ecology perspective, we conclude that estuaries—even ones that are over-depleted and succumbing to human impacts—are paramount environments for the Guiana dolphin, serving as important sources of prey for the species and other sympatric marine mammals.