Individual free fatty acids (FAs) play important roles in metabolic homeostasis, many through engagement with more than 40G protein–coupled receptors. Searching for receptors to sense beneficial omega-3 FAs of fish oil enabled the identification of GPR120, which is involved in a spectrum of metabolic diseases. Here, we report six cryo–electron microscopy structures of GPR120 in complex with FA hormones or TUG891 and G _i or G _iq trimers. Aromatic residues inside the GPR120 ligand pocket were responsible for recognizing different double-bond positions of these FAs and connect ligand recognition to distinct effector coupling. We also investigated synthetic ligand selectivity and the structural basis of missense single-nucleotide polymorphisms. We reveal how GPR120 differentiates rigid double bonds and flexible single bonds. The knowledge gleaned here may facilitate rational drug design targeting to GPR120.