Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a growing family of bioactive peptides. Among RiPPs, the bacterial toxin polytheonamide A is characterized by a unique set of post-translational modifications catalyzed by novel radical S-adenosyl-l-methionine (SAM) enzymes. Here we show that the radical SAM enzyme PoyD catalyzes in vitro polytheonamide epimerization in a C-to-N directional manner. By combining mutagenesis experiments with labeling studies and investigating the enzyme substrate promiscuity, we deciphered in detail the mechanism of PoyD. We notably identified a critical cysteine residue as a likely key H atom donor and demonstrated that PoyD belongs to a distinct family of radical SAM peptidyl epimerases. In addition, our study shows that the core peptide directly influences the epimerization pattern allowing for production of peptides with unnatural epimerization patterns.