An 84-year-old man visited our hospital with epigastralgia.Levels of hepatic and biliary enzymes and CRP were elevated, as detected by a blood test.On a CT scan, a swollen gallbladder with stones was detected.The patient was admitted to the hospital with a diagnosis of Grade I acute cholecystitis.Conservative treatment was continued with antibiotic administration and the patient was discharged from the hospital with improvement on day 6 after admission.Three months later, the patient underwent laparoscopic cholecystectomy.In the gallbladder, a 45×45 mm tumor was found.Upon pathological examination, diffuse proliferation of lymphocyte-like heterotypic cells and subserosal invasion were observed.Immunohistochemistry results were negative for MUM1 and positive for CD10 and Bcl6 markers.A malignant diffuse large B-cell lymphoma was diagnosed.We experienced a case of malignant lymphoma of the gallbladder diagnosed after surgery for acute cholecystitis, which we herein report with literature consideration.