A 55-year-old man was admitted to a local hospital due to fever, diarrhea, and gait disturbance. He was diagnosed with pneumonia and treated with antibiotics. The following day he developed neurological symptoms, including unconsciousness, and was transferred to our hospital. Hyponatremia, rhabdomyolysis, liver dysfunction, and neurological symptoms were observed, so we suspected pneumonia caused by Legionella pneumophila and administered fluoroquinolone and macrolide. Brain magnetic resonance imaging (MRI) (diffusion-weighted imaging) revealed a high-intensity region in the splenium of the corpus callosum. Alongside an improvement in pneumonia symptoms, his neurological symptoms and MRI abnormalities normalized. Legionella pneumophila serogroup 2 was cultured from bronchoalveolar lavage fluid, and he was diagnosed with a reversible splenial lesion associated with pneumonia caused by Legionella pneumophila.