Apelin is the endogenous ligand of the APJ receptor, a member of the G‐protein‐coupled receptor family. The apelin–APJ complex has been detected in many tissues and is emerging as a promising target for several pathophysiological conditions. There is currently little information on the structure–activity relationship (SAR) of the apelin hormone. In an effort to better delineate SAR, we synthesized analogues of apelin‐13 modified at selected positions with unnatural amino acids, with a particular emphasis on the C‐terminal portion. Analogues were then tested in binding and functional assays by evaluating G_i/o‐mediated decreases in cAMP levels and by assessing β‐arrestin2 recruitment to the APJ receptor. The plasma stability of new compounds was also assessed. Several analogues were found to possess increased binding and higher stability than the parent peptide.