Helicobacter pylori-induced gastritis predisposes to the development of gastric cancer. Increased epithelial tight junction permeability and alterations in apical-junctional complexes are also associated with an increased risk of carcinogenesis. Phosphorylation of myosin regulatory light chain (MLC) by MLC kinase (MLCK) regulates tight junction function. We determined whether MLCK was activated by H pylori and defined the mechanisms through which such activation dysregulates gastric epithelial barrier function. MKN28 gastric epithelial cells were cocultured with the H pylori cag(+) strain 60190 or cagA(-), cagE(-), ureB(-), or vacA(-) mutants. MLC phosphorylation and barrier integrity were determined by immunoblot analysis and transepithelial electrical resistance measurements, respectively. Localization of the tight junction protein occludin was determined by immunocytochemistry in MKN28 cells and INS-GAS mice. H pylori induced a progressive loss of barrier function that was attenuated by inactivation of ureB, but not cagA, cagE, or vacA. Reductions in transepithelial electrical resistance were also dependent on functional urease activity. H pylori increased MLC phosphorylation in epithelial monolayers; this was significantly decreased by inhibition of MLCK or Rho kinase or by loss of UreB. H pylori infection of either cultured monolayers or hypergastrinemic INS-GAS mice induced occludin endocytosis, reflecting cytoskeletally mediated disruption of tight junctions. H pylori increases MLC phosphorylation, occludin internalization and barrier dysfunction in gastric epithelial cells. This process requires functional urease activity and is independent of the cag pathogenicity island or VacA. These data provide new insights into the mechanisms by which H pylori disrupts gastric barrier function.