Dieulafoy lesions are rare vascular malformations of the gastrointestinal tract; however, they can lead to fatal vascular bleeding. Immunoglobulin G4-related disease (IgG4-RD) is a rare systemic fibroinflammatory disease involving multiple organs, including the vasculature. To date, no autopsy reports of Dieulafoy lesions with IgG4-RD have been described in the literature. A 48-year-old man was found dead in his home with hematochezia. Postmortem computed tomography revealed high-density gastric contents and an enlarged iso-density area in the pancreas, indicating gastric hemorrhage and mass-forming lesions. Macroscopic and histological examinations revealed an ulcer of the body of the stomach with a large amount of hemorrhage from the enlarged artery in the submucosal layer, confirming the rupture of the Dieulafoy lesion. Moreover, lymphocyte infiltrations with increased IgG4 positive cells were found in the pancreas, thyroid gland, and arteries in non-ulcer regions of the stomach, suggesting IgG4-RD. Serum biochemical analysis showed elevated levels of inflammatory mediators, such as IgE, soluble-interleukin-2 receptor, and C-reactive protein. These findings suggest that systemic inflammation caused by IgG4-RD could, at least in part, contribute to the development of Dieulafoy lesions and fatal rupture of the lesion. This case report highlights the importance of autopsy research focusing on Dieulafoy lesions and IgG4-RD to promote awareness and a better understanding of the relationships between these treatable diseases to establish earlier and effective interventional strategies for better patient outcomes.