Human complement regulators are important targets for pathogenic microorganisms. In one such interaction, Bordetella pertussis binds human C4b‐binding protein (C4BP), a high‐molecular‐weight plasma protein that acts as inhibitor of the classical pathway of complement activation. At least two different B. pertussis surface components, one of which is the virulence factorfilamentous hemagglutinin (FHA), contribute to the binding. We used a set of C4BP mutants and monoclonal antibodies to characterize the region in C4BP that binds B. pertussis and analyzed the salt sensitivity of the interaction. These studies indicated that positively charged residues at the interface between complement control protein modules 1–2 in the C4BP α‐chain are important for binding, and that the site in C4BP that binds B. pertussis is very similar, but not identical, to the C4b‐binding site. Bacteria‐bound C4BP retained its complement regulatory functionand B. pertussis selectively bound C4BP in human plasma, indicating that binding occurs also in vivo. Together, these findings indicate that B. pertussis exploits a site in C4BP, resembling that used by the natural ligand C4b.