Ligand-binding assays are the linchpin of drug discovery and medicinal chemistry. Cell-surface receptors and their ligands have traditionally been characterized by radio-ligand-binding assays, which have low temporal and spatial resolution and entail safety risks. Here, we report a powerful alternative (GlycoFRET), where terbium-labeled fluorescent reporters are irreversibly attached to receptors by metabolic glycan engineering. For the first time, we show time-resolved fluorescence resonance energy transfer between receptor glycans and fluorescently labeled ligands. We describe G1ycoFRET for a GPI-anchored receptor, a G-protein-coupled receptor, and a heterodimeric cytokine receptor in living cells with excellent sensitivity and high signal-to-background ratios. In contrast to previously described methods, G1ycoFRET does not require genetic engineering or antibodies to label receptors. Given that all cell-surface receptors are glycosylated, we expect that G1ycoFRET can be generalized with applications in chemical biology and biotechnology, such as target engagement, receptor pharmacology, and high-throughput screening.