Various peptides found in nature, such as cyclotides and conotoxins, bear complex multicyclic structures and have been shown to have a wide variety of biological activites1. Thus, a technology to produce large libraries of multicyclic peptides could have potential for the screening or in vitro selection of novel peptide drug candidates. To this end, we report a new method of synthesizing tricyclic peptides with a fused topology via a reprogrammed genetic code and chemical modification2. Genetic code reprogramming using the FIT system3 allows us to ribosomally synthesize N-terminally chloroacetylated peptides which can spontaneously form thioether bonds with downstream cysteines to form monocyclic peptides. Previous characterization has shown that this N-terminal chloroacetyl group selectively forms thioether bonds with the closest cysteine in an amino acid sequence but is not able to form a thioether bond with an adjacent cysteine4. By taking advantage of this unique selectivity, a peptide with an N-terminal chloroacetyl group and four downstream cysteines, one in the second amino acid position, will spontaneously form a thioether bond with the N-terminus and the second cysteine allowing the remaing three cysteines to be further cyclized by the addition of tris(bromomethyl)benzene to result in a fused tricyclic peptide. We have shown that this method is compatible with peptides of various lengths and amino acid compositions and results in near quantitative conversion with little to no side products.