AbstractImmunoglobulin G4‐related disease is a fibroinflammatory condition of unclear etiology that can present with inflammatory changes and enlargement of a wide variety of organs, most commonly in the gastrointestinal tract. A diagnosis requires an elevated serum immunoglobulin G4 concentration and a tissue biopsy showing a dense plasma cell infiltrate with an increased percentage of immunoglobulin G4+ plasma cells. This disease infrequently presents in the genitourinary tract, and as such might be unfamiliar to and potentially overlooked by urologists. Here we present the third reported case of immunoglobulin G4‐related disease manifesting as a mass in the urinary bladder.