Autotransporters are a distinct class of bacterial outer membrane proteins (OMPs) with a specific domain architecture and transmembrane topology. A domain composed of a 12-stranded β-barrel is folded and inserted into the outer membrane to facilitate presentation of the covalently linked passenger domain to the bacterial cell surface. Once translocated and folded into a characteristic, elongated β-helix, passenger domains play key roles in pathogenesis; some passengers remain attached to their β-barrels to function as adhesins while others are processed and released into the environment to function as toxins, or in disruption of the host immune response. There is general agreement that the passenger domain is translocated in a C- to N-terminal direction where the C-terminus folds into a stable, protease-resistant structure. It has been proposed that folding of the portion of the passenger domain that appears first at the cell surface into a stable scaffold nucleates processive folding of the more N-terminal segments of the passenger domain during translocation. However, many mechanistic details are lacking. Using biochemical and biophysical methods, we have unraveled the molecular basis for the folding of β-helical autotransporter passenger domains into their functional form. This work demonstrates that the autotransporter b-barrel domain is a folding vector and a potential drug target for the prevention and treatment of autotransporter-mediated infectious diseases.