Immunoglobulin G4 (IgG4)-related disease (IgG4RD) is a relatively recently recognized entity that is histopathologically characterized by an extensive infiltration of lymphocytes and IgG4-positive plasma cells with dense fibrosis. IgG4RD is now known to affect any organ system, and a few cases of gastrointestinal lesions have also been reported. However, solitary IgG4RD of the stomach is still very rare. Furthermore, as it can mimic malignant conditions, it is important to recognize this disease to avoid unnecessary surgery. Herein, we present a case of IgG4RD presenting as an isolated subepithelial mass in the stomach.