We herein report the case of a 49-year-old man with clinically mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) associated with Legionnaires' disease due to Legionella pneumophila serogroup 2. Past reports suggest that Legionella infection is frequent in cases of MERS-associated pneumonia. Obtaining an early diagnosis of legionella infection is a challenge, especially if a Legionella pneumophila serogroup other than serogroup 1 contains the causative agent. In this case, the splenial lesion played an important role in recognizing the legionella infection. We suggest that legionella infection should be considered as a differential diagnosis in cases of splenial lesions associated with pneumonia.