Enterohaemorrhagic Escherichia coli attaches to the intestine through actin pedestals that are formed when the bacterium injects its protein EspF_U (also known as TccP) into host cells. EspF_U potently activates the host WASP (Wiskott–Aldrich syndrome protein) family of actin-nucleating factors, which are normally activated by the GTPase CDC42, among other signalling molecules. Apart from its amino-terminal type III secretion signal, EspF_U consists of five-and-a-half 47-amino-acid repeats. Here we show that a 17-residue motif within this EspF_U repeat is sufficient for interaction with N-WASP (also known as WASL). Unlike most pathogen proteins that interface with the cytoskeletal machinery, this motif does not mimic natural upstream activators: instead of mimicking an activated state of CDC42, EspF_U mimics an autoinhibitory element found within N-WASP. Thus, EspF_U activates N-WASP by competitively disrupting the autoinhibited state. By mimicking an internal regulatory element and not the natural activator, EspF_U selectively activates only a precise subset of CDC42-activated processes. Although one repeat is able to stimulate actin polymerization, we show that multiple-repeat fragments have notably increased potency. The activities of these EspF_U fragments correlate with their ability to coordinate activation of at least two N-WASP proteins. Thus, this pathogen has used a simple autoinhibitory fragment as a component to build a highly effective actin polymerization machine.