Biased ligands of G protein-coupled receptors (GPCRs) may have improved therapeutic benefits and safety profiles. However, the molecular mechanism of GPCR biased signaling remains largely unknown. Using apelin receptor (APJ) as a model, we systematically investigated the potential effects of amino acid residues around the orthosteric binding site on biased signaling. We discovered that a single residue mutation I109A (I1093.32) in the transmembrane domain 3 (TM3) located in the deep ligand-binding pocket was sufficient to convert a balanced APJ into a G protein signaling biased receptor. APJ I109A mutant receptor retained full capabilities in ligand binding and G protein activation, but was defective in GRK recruitment, β-arrestin recruitment, and downstream receptor-mediated ERK activation. Based on molecular dynamics simulations, we proposed a molecular mechanism for biased signaling of I109A mutant receptor. We postulate that due to the extra space created by I109A mutation, the phenyl group of the last residue (Phe-13) of apelin rotates down and initiates a cascade of conformational changes in TM3. Phe-13 formed a new cluster of hydrophobic interactions with the sidechains of residues in TM3, including F1103.33 and M1133.36, which stabilizes the mutant receptor in a conformation favoring biased signaling. Interruption of these stabilizing interactions by double mutation F110A/I109A or M113A/I109A largely restored the β-arrestin-mediated signaling. Taken together, we describe herein the discovery of a biased APJ mutant receptor and provide detailed molecular insights into APJ signaling selectivity, facilitating the discovery of novel therapeutics targeting APJ.