A 60-year-old man was admitted because of fever, headache, and difficulty in walking. Respiratory symptoms included only mild cough, but crackles were present on auscultation at the right lung base, the chest roentgenogram and computed tomography scans showed consolidation in the right lower lobe. Laboratory findings revealed hyponatremia, elevated liver function test values and creatine phosphokinase, and Legionella pneumophila antigen in urine. Neurological examination revealed mild mental status change, dysmetria, dysarthria, and ataxic gait. Diffusion-weighted magnetic resonance imaging (MRI) of the brain, performed at the time of admission, revealed regions of high intensity in the splenium corpus callosum. We diagnosed Legionnaires' pneumonia accompanied by clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS), and started treatment with ciprofloxacin and methylprednisolone at 1 mg/kg/day. Neurological symptoms gradually improved. On day 6 after admission, mild dysarthria and ataxic gait remained, a ¹²³I-IMP single photon emission computed tomography revealed no abnormality. On day 15 after admission, the only neurological symptom was mild ataxic gait; the MRI scans showed no abnormalities. On day 29 after admission, neurological symptoms were completely resolved. This is the first reported case of Legionnaires' pneumonia accompanied by clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) was treated with not only antibiotics but also corticosteroid.