Modern agricultural practices rely on herbicides to reduce yield losses. Herbicide-resistant weeds threaten herbicide utility and, hence, food security. New herbicide modes of action and integrated pest-management practices are vital to mitigate this threat. As the antimalarials that target the bifunctional enzyme dihydrofolate reductase-thymidylate synthase (DHFR-TS) have been shown to be herbicidal, DHFR-TS might represent a mode-of-action target for the development of herbicides. Here, we present the crystal structure of a DHFR-TS (AtDHFR-TS1) from the model dicot Arabidopsis thaliana. It shows a divergent DHFR active site and a linker domain that challenges previous classifications of bifunctional DHFR-TS proteins. This plant-conserved architecture enabled us to develop highly selective herbicidal inhibitors of AtDHFR-TS1 over human DHFR and identify inhibitors with unique scaffolds via a large-library virtual screen. These results suggest that DHFR-TS is a viable herbicide target.