Autoimmune pancreatitis (AIP) is a recently recognized disease entity. In some patients, this disease is associated with other inflammatory diseases. In this study, we aimed to elucidate the pathologic characteristics of AIP-associated gastritis (AIP-G). We evaluated and compared the pathologic findings and immunohistochemical expressions of immunoglobulin G (IgG)4 and IgG in gastric biopsy specimens from 13 AIP-G patients with those from patients of 2 control groups. We divided the AIP-G patients who did not receive steroid therapy [AIP-G-ST(-)] into the following 2 groups: without Helicobacter pylori (HP) infection [AIP-G-HP(-)] and with HP infection [AIP-G-HP(+)]. The control groups comprised 19 patients who were diagnosed with chronic active gastritis associated with HP infection and 7 patients with nonsteroidal anti-inflammatory drug-induced gastritis. We classified the findings for the gastric mucosa into those for the upper and the lower lamina propria. The characteristic finding of AIP-G groups was diffusely lymphoplasmacytic infiltration in the lamina propria. The IgG4-positive plasma cell/IgG-positive plasma cell ratios (IgG4/IgG ratios) in both the upper and lower lamina propria in the AIP-G-ST(-) groups were predominantly higher than the corresponding values in the other groups. In the AIP-G-ST(-) groups, the IgG4/IgG ratio in the lower lamina propria was predominantly higher than that in the upper lamina propria, irrespective of the HP status. In conclusion, diffuse lymphoplasmacytic infiltration in the lamina propria and increased IgG4/IgG ratio in the gastric mucosa (notably in the lower lamina propria) may be the characteristic findings of AIP-G.