Mild encephalitis/encephalopathy with reversible splenial lesion (MERS) has not been described in lymphoma patients. A 65-year-old man with refractory mantle cell lymphoma (MCL) presented typical MRI features of MERS. The patient's cerebrospinal fluid contained an increased number of reactive T-cells; a small number of MCL cells were detected by immunoglobulin heavy chain-polymerase chain reaction (IGH-PCR). His symptoms and the splenial lesion resolved in response to ibrutinib treatment, although the patient eventually died of progressive MCL with overt leptomeningeal disease. We suggest that central nervous system involvement in MCL can present clinicoradiological features of MERS and that ibrutinib could be a choice of treatment.