Abstract Colibactin is a gut microbiome metabolite of unknown structure that has been implicated in colorectal cancer formation. Several studies now suggest that the tumorgenicity of colibactin derives from interstrand cross-linking of host DNA. Here we use a combination of genetics, isotope labeling, tandem MS, and chemical synthesis to deduce the structure of colibactin. Our structural assignment accounts for all known biosynthetic data and suggests roles for the final unaccounted enzymes in the colibactin gene cluster. DNA cross-link degradation products derived from synthetic and natural colibactin were indistinguishable by tandem MS analysis, thereby confirming the structure prediction. This work reveals the structure of colibactin, which has remained incompletely defined for over a decade.