A major challenge in ecology is to understand broadscale trends in the impact of environmental change. We provide the first integrative analysis of the effects of eutrophication on plants, herbivores, and their interactions in coastal wetlands across latitudes. We show that fertilisation strongly increases herbivory in salt marshes, but not in mangroves, and that this effect increases with increasing latitude in salt marshes. We further show that stronger nutrient effects on plant nitrogen concentration at higher latitudes is the mechanism likely underlying this pattern. This biogeographic variation in nutrient effects on plant–herbivore interactions has consequences for vegetation, with those at higher latitudes being more vulnerable to consumer pressure fuelled by eutrophication. Our work provides a novel, mechanistic understanding of how eutrophication affects plant–herbivore systems predictably across broad latitudinal gradients, and highlights the power of incorporating biogeography into understanding large-scale variability in the impacts of environmental change.