We describe the discovery and structure of an undecapeptide natural product from a marine sponge, termed halichondamide A, that is morphed into a fused bicyclic ring topology via two disulfide bonds. Molecular dynamics simulations allow us to posit that the installation of one disulfide bond biases the intermediate peptide conformation and predisposes the formation of the second disulfide bond. The natural product was found to be mildly cytotoxic against liver and breast cancer cell lines.