AbstractAmino acid modifications are essential for the structural diversity and bioactivity of ribosomally synthesized and post‐translationally modified peptide natural products (RiPPs). A particularly large and virtually untapped pool of unusual RiPPs and associated modifying enzymes is provided by uncultivated bacteria. An example is the chemically rich sponge symbiont “Candidatus Entotheonella factor”, which produces the hypermodified polytheonamides of the poorly studied proteusin RiPP family. In addition to the polytheonamide genes, “E. factor” contains several further additional RiPP clusters of unknown function. Here we provide insights into one of these cryptic proteusin pathways by identifying an enzyme (PtyS) that catalyzes the S‐methylation of cysteine residues. S‐methylcysteine is rare in natural peptides and proteins, and the enzymatic activity was previously unknown for RiPPs, thus adding a new modification to the ribosomal peptide toolbox.