Mild encephalitis/encephalopathy with a reversible splenial lesion syndrome (MERS) is a rare clinico-radiological entity. Its occurrence after Rituximab (RTX) has never been described. We report the first case of MERS following RTX in a patient treated for IgG4 related disease (IgG4-RD). It was about a 33-year-old patient diagnosed since 2017, with an IgG4-RD. This diagnosis was made in front of a prolonged fever, sicca syndrome, hepatic damage and renal pseudotumor associated to a high level of IGg4 at 2.8 g/l with suggestive renal histology. The patient was treated with corticosteroid therapy with persistence of renal impairment and nephrotic syndrome that’s why RTX has been indicated. The patient received his first dose of RTX and presented few hours after, neurological and respiratory impairments. An infectious investigation comprising a SARS CoV-2 PCR and viral PCRs (VZV, Herpes and CMV) on cerebrospinal fluid (CSF) were negative. The HBV, HCV, HIV, Parvo B19, CMV, EBV, Herpes, Mycoplasma and syphilis serologies as well as Legionella antigenuria were also negative. The patient had received methylprednisone boli, intravenous immunoglobulin’s associated with sodium valproate with good evolution. The diagnosis of MERS induced by RTX is likely. However the viral aetiology is not excluded.