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GPCR functional selectivity opens new opportunities for the design of safer drugs. Ligands orchestrate GPCR signaling cascades by modulating the receptor conformational landscape. Our study provides insights into the dynamic mechanism enabling opioid ligands to preferentially activate the G protein over the β-arrestin pathways through the μ-opioid receptor (μOR). We combine functional assays in living cells, solution NMR spectroscopy, and enhanced-sampling molecular dynamic simulations to identify the specific μOR conformations induced by G protein-biased agonists. In particular, we describe the dynamic and allosteric communications between the ligand-binding pocket and the receptor intracellular domains, through conserved motifs in class A GPCRs. Most strikingly, the biased agonists trigger μOR conformational changes in the intracellular loop 1 and helix 8 domains, which may impair β-arrestin binding or signaling. The findings may apply to other GPCR families and provide key molecular information that could facilitate the design of biased ligands. [Display omitted] • NMR and MD simulations help define the molecular mechanisms of μOR-biased signaling • Biased, unbiased, and partial agonists stabilize different μOR conformations • Biased agonists stabilize specific conformations in the TM7, ICL1, and H8 domains • The bias in conformation persists after binding to a G protein mimetic nanobody Biased ligands of GPCRs offer new drug design strategies to enhance beneficial drug actions while reducing side effects. Cong et al. combined molecular simulations, NMR spectroscopy, and functional assays to uncover the molecular mechanism of ligand bias in the μ-opioid receptor, which provides structural basis for designing better opioid analgesics. [ABSTRACT FROM AUTHOR]