The relaxin family peptides play a variety of biological functions by activating four G protein-coupled receptors, RXFP1-4. Among them, insulin-like peptide 5 (INSL5) and relaxin-3 share the highest sequence homology, but they have distinct receptor preference: INSL5 can activate RXFP4 only, while relaxin-3 can activate RXFP3, RXFP4, and RXFP1. Previous studies suggest that the A-chain is responsible for their different selectivity for RXFP1. However, the mechanism by which INSL5 distinguishes the homologous RXFP4 and RXFP3 remains unknown. In the present work, we chemically evolved INSL5 in vitro to a strong agonist of both RXFP4 and RXFP3 through replacement of its five B-chain residues with the corresponding residues of relaxin-3. We identified four determinants (B2Glu, B9Leu, B17Tyr, and a rigid B-chain C-terminus) on INSL5 that are responsible for its inactivity at RXFP3. In reverse experiments, we grafted these determinants onto a chimeric R3/I5 peptide, which contains the B-chain of relaxin-3 and the A-chain of INSL5, and retains full activation potency at RXFP3 and RXFP4. All resultant R3/I5 mutants retained high activation potency towards RXFP4, but most displayed significantly decreased or even abolished activation potency towards RXFP3, confirming the role of these four INSL5 determinants in distinguishing RXFP4 from RXFP3.