We report the rare case of a 38-year-old man who suffered from aseptic meningitis. Brain magnetic resonance imaging showed an ovoid lesion with a high signal intensity on T2- and diffusion-weighted images in the splenium of the corpus callosum that completely disappeared within a week. There were no symptoms or signs associated with the lesion, and the prognosis was good. Although rare in adult patients, awareness of a reversible splenial lesion in meningoencephalitis is important.