A 56-year-old woman with rheumatoid arthritis treated with methotrexate (MTX) was admitted to our hospital due to multiple liver tumors. Contrast-enhanced computed tomography (CT) revealed multiple hypovascular masses, and 18F-fluorodeoxyglucose positron emission tomography CT showed diffuse abnormal accumulation in the liver only. We therefore made a diagnosis of MTX-related primary hepatic lymphoma (MTX-PHL) exhibiting features of diffuse large B-cell lymphoma. Although MTX has been reported to increase the risk of lymphoproliferative disorders, MTX-PHL has not been reported previously. The present case is the first case in which MTX appears to have been involved in the development of PHL.