Genomic and metagenomic investigations have recently led to the delineation of a novel class of natural products called ribosomally synthesized and post-translationally modified peptides (RiPPs). RiPPs are ubiquitous among living organisms and include pharmaceutically relevant compounds such as antibiotics and toxins. A prominent example is polytheonamide A, which exhibits numerous post-translational modifications, some of which were unknown in ribosomal peptides until recently. Among these post-translational modifications, C-methylations have been proposed to be catalyzed by two putative radical S-adenosylmethionine (rSAM) enzymes, PoyB and PoyC. Here we report the in vitro activity of PoyC, the first B-12-dependent rSAM enzyme catalyzing peptide C-beta-methylation. We show that PoyC catalyzes the formation of S-adenosylhomocysteine and 5'-deoxyadenosine and the transfer of a methyl group to L-valine residue. In addition, we demonstrate for the first time that B-12-rSAM enzymes have a tightly bound MeCbl cofactor that during catalysis transfers a methyl group originating from S-adenosyl-L-methionine. Collectively, our results shed new light on polytheonamide biosynthesis and the large and emerging family of B-12-rSAM enzymes.