Diffuse large B-cell lymphoma (DLBCL) is the most common type of non-Hodgkin lymphoma. The T-cell marker CD5 is positive in 5% to 10% of cases of DLBCL. We describe a rare case of CD5-positive DLBCL presenting with swelling of the mandibular gingiva. The patient was a 78-year-old-woman with a chief complaint of swelling of the mandibular gingiva. Biopsy was performed, and the patient was given a diagnosis of DLBCL. Moreover, analysis of the immunophenotype demonstrated that the lymphoma cells expressed CD5. The final diagnosis was CD5-positive DLBCL. The patient was treated with R-CHOP. There have been no signs of recurrence after the chemotherapy.