We present here a cyclic peptide ligand, cy(WQETR), that binds to the terbium ion (Tb3+) and enhances Tb3+ luminescence intensity through the antenna effect. This peptide was identified through screening a cyclic peptide library against Tb3+ with an apparent EC50 of 540 μM. The tryptophan residue from the peptide directly interacts with the Tb3+ ion, which provides access to a low-lying triplet excited state of the tryptophan. Direct excitation of this triplet state enables energy transfer to the Tb3+ ion and enhances Tb3+ luminescence intensity by 150 fold. We further showcase the application of this cy(WQETR)-Tb3+ system by demonstrating the detection of tromethamine with a detection limit of 0.5 mM.