Immunoglobulin (Ig)G4-related sclerosing disease is a fibroinflammatory disorder characterized by tumor-forming lesions at multiple anatomic sites and by increased serum levels of IgG4. IgG4-related lymphadenopathy, defined as lymphadenopathy developing in patients with IgG4-related sclerosing disease, is known to manifest in 5 histologic patterns: (1) multicentric Castleman disease-like; (2) reactive follicular hyperplasia; (3) interfollicular plasmacytosis with immunoblasts; (4) progressive transformation of germinal centers-like; and (5) inflammatory pseudotumor-like. Herein, we describe a 37-year-old man with an additional pattern of IgG4-related lymphadenopathy that we designate as infectious mononucleosis-like. This pattern is characterized by effacement of the nodal architecture by an infiltrate composed of numerous mature plasma cells, plasmacytoid cells, large basophilic transformed lymphocytes (immunoblasts), and small-sized to medium-sized lymphocytes and histiocytes. Perivascular fibrosis and karyorrhectic debris with fibrin deposition were also focally identified. Epstein-Barr virus-encoded small RNA in situ hybridization showed scattered positive small lymphocytes, 1% to 2%. The initial spike of IgG4 in serum (>4400 mg/dL) decreased by half after 1 month of steroid therapy. His condition was stable during 1 year of follow-up. We report this case because the findings expand the morphologic spectrum of IgG4-related lymphadenopathy.