© The Author(s) 2025.Omnivory is increasingly recognized as a dynamic stabilizing force under environmental change. Despite its ubiquity across ecosystems, trophic levels and spatiotemporal scales, our empirical understanding of how omnivores respond to changing conditions in terrestrial ecosystems is limited. Here we combine macroecological and paleoecological approaches across seven bear species—the largest terrestrial carnivores—and discover they dynamically adapt their trophic position in food webs to resource availability. Throughout their ranges, bears shift to carnivory in unproductive ecosystems with short growing seasons and to herbivory in productive ecosystems with long growing seasons. In line with this, isotopic evidence from the Late Pleistocene and Holocene reveals a sharp decrease in the trophic position of the European brown bear in response to increasing net primary productivity and growing season length. These findings reveal a mechanism of trophic rewiring that alters the functional role of large carnivores in ecosystems and may simultaneously stabilize food web dynamics under global change.