Nicotinamide -methyltransferase (NNMT), which catalyzes the methylation of nicotinamide, is a cytosolic enzyme that has attracted much attention as a therapeutic target for a variety of diseases. However, despite the considerable interest in this target, reports of NNMT inhibitors have still been limited to date. In this work, utilizing translated macrocyclic peptide libraries, we identified peptide as a novel class of NNMT inhibitors. Further exploration based on the X-ray cocrystal structures of the peptides with NNMT provided a dramatic improvement in inhibitory activity (peptide : IC = 0.15 nM). Furthermore, by balance of the peptides' lipophilicity and biological activity, inhibitory activity against NNMT in cell-based assay was successfully achieved (peptide : cell-based IC = 770 nM). These findings illuminate the potential of cyclic peptides as a relatively new drug discovery modality even for intracellular targets.