Helicobacter pylori cagA-positive strains are associated with gastritis, ulcerations and gastric adenocarcinoma. CagA is delivered into gastric epithelial cells and, on tyrosine phosphorylation, specifically binds and activates the SHP2 oncoprotein, thereby inducing the formation of an elongated cell shape known as the 'hummingbird' phenotype. In polarized epithelial cells, CagA also disrupts the tight junction and causes loss of apical-basolateral polarity. We show here that H. pylori CagA specifically interacts with PAR1/MARK kinase, which has an essential role in epithelial cell polarity. Association of CagA inhibits PAR1 kinase activity and prevents atypical protein kinase C (aPKC)-mediated PAR1 phosphorylation, which dissociates PAR1 from the membrane, collectively causing junctional and polarity defects. Because of the multimeric nature of PAR1 (ref. 14), PAR1 also promotes CagA multimerization, which stabilizes the CagA-SHP2 interaction. Furthermore, induction of the hummingbird phenotype by CagA-activated SHP2 requires simultaneous inhibition of PAR1 kinase activity by CagA. Thus, the CagA-PAR1 interaction not only elicits the junctional and polarity defects but also promotes the morphogenetic activity of CagA. Our findings revealed that PAR1 is a key target of H. pylori CagA in the disorganization of gastric epithelial architecture underlying mucosal damage, inflammation and carcinogenesis.