The human colon is colonized by trillions of bacteria that play substantial roles in human health and disease1. Epidemiological and experimental studies suggest that certain colonic bacteria can stimulate the development and progression of colorectal cancer2. One such bacterium, enterotoxigenic Bacteroides fragilis, drives colon tumour formation through the action of a single toxin, the B. fragilis toxin (BFT)3,4. BFT is a metalloprotease that binds to a colonic epithelial cell receptor and causes cleavage of the E-cadherin ectodomain, leading to epithelial barrier disruption, inflammation and increased cellular proliferation4-6. However, the identity of the BFT receptor is unknown and the molecular mechanism of BFT-initiated E-cadherin cleavage is not well understood. Here we identify claudin-4 as a BFT receptor through a genome-wide CRISPR screen and demonstrate that claudin-4 binding promotes BFT-mediated cleavage of cell surface E-cadherin. Our work both sheds light on BFT's mechanism of action and opens avenues for the development of anti-BFT therapies, which may prove useful for colorectal cancer prevention and treatment of acute enterotoxigenic B. fragilis infection.