Background <p>Immunoglobulin G4 (IgG4)-related gastrointestinal diseases (IgG4-GID) are becoming increasingly recognized. However, few cases have been reported, and the disease concept is not yet well established. This study aimed to elucidate the clinical features of IgG4-GID.</p> Methods <p>This nationwide multicenter retrospective study collected 37 cases of IgG4-GID, which were classified and analyzed based on the pathological findings and the presence or absence of IgG4-related diseases in other organs. The pathological possibility of IgG4-GID was classified as definite, highly likely, probable, or unlikely based on the presence of typical pathological findings, number of IgG4-positive cells, and adequacy of histological evaluation.</p> Results <p>Thirteen patients were classified as unlikely to undergo pathological evaluation. Among the remaining 24 cases, 20 had other organ involvement (pathologically definite, <i>n</i> = 8; highly likely, <i>n</i> = 6; probably, <i>n</i> = 6). The four cases without the involvement of other organs were classified as definite. After defining definite and highly likely cases, 18 cases of IgG4-GID were identified. The most commonly affected organs were the stomach (<i>n</i> = 12) and the duodenum (<i>n</i> = 6), with one case involving both. Endoscopic findings most frequently showed ulcers (<i>n</i>  = 7), followed by submucosal tumor (SMT)-like morphology (<i>n</i>  = 6). Treatments included surgery (<i>n</i>  = 8; SMT-like cases), steroids (<i>n</i>  = 2), and proton pump inhibitors or potassium-competitive acid blockers (<i>n</i>  = 3), with all cases showing improvement.</p> Conclusions <p>IgG4-GID exhibits characteristic pathological findings and various endoscopic features. Although many patients respond to treatment, some undergo surgery; thus, increased awareness may help avoid unnecessary surgeries.</p>