Microviridins and other ω-ester-linked peptides, collectively known as graspetides, are characterized by side-chain-side-chain linkages installed by ATP-grasp enzymes. Here we report the discovery of a family of graspetides, the gene clusters of which also encode an O-methyltransferase with homology to the protein repair catalyst protein L-isoaspartyl methyltransferase. Using heterologous expression, we produced fuscimiditide, a ribosomally synthesized and post-translationally modified peptide (RiPP). NMR analysis of fuscimiditide revealed that the peptide contains two ester cross-links forming a stem-loop macrocycle. Furthermore, an unusually stable aspartimide moiety is found within the loop macrocycle. We fully reconstituted fuscimiditide biosynthesis in vitro including formation of the ester and aspartimide moieties. The aspartimide moiety embedded in fuscimiditide hydrolyses regioselectively to isoaspartate. Surprisingly, this isoaspartate-containing peptide is also a substrate for the L-isoaspartyl methyltransferase homologue, thus driving any hydrolysis products back to the aspartimide form. Whereas an aspartimide is often considered a nuisance product in protein formulations, our data suggest that some RiPPs have aspartimide residues intentionally installed via enzymatic activity.