We report a case of primary central nervous system lymphoma (PCNSL) in an 81-year-old man who had undergone radical cystectomy with an ileal conduit urostomy due to a diagnosis of muscle-invasive bladder cancer. The postoperative diagnosis was invasive urothelial carcinoma (pT2bN1M0, stage IV). Gemcitabine-cisplatin therapy was provided as adjuvant chemotherapy, and there was no recurrence during follow-up. Four years after surgery, he visited the emergency department because of weakness of the lower extremities and stuttering. He was found to have a parietal lobe mass on magnetic resonance imaging (MRI) and hospitalized with suspicion of brain metastasis. Despite examination by a neurosurgeon, it was not possible to make a clinical diagnosis, and the patient gradually deteriorated and died 21 days later. The pathology results were diagnostic of PCNSL.