Diffuse large B-cell lymphoma (DLBCL) is a common cancer in haematology. We report a case of DLBCL mimicking malignant pleural mesothelioma (MPM). A 75-year-old man with a 1-week exacerbation of dyspnoea on exertion and right pleural effusion on chest radiography was admitted to our hospital. Positron emission tomography/computed tomography revealed diffuse pleural thick lesions and massive pleural effusion, and these lesions had accumulation of fluorodeoxyglucose. Because we suspected MPM or lung cancer, we performed a biopsy with thoracoscopy to confirm the diagnosis. A biopsy of the pleural effusion with thoracoscopy revealed DLBCL. Chemotherapy was immediately selected, and the diffuse thickened pleural wall and massive pleural effusion of the right chest were significantly improved after two cycles of chemotherapy.