Acute Disseminated Encephalomyelitis (ADEM) is a monophasic, inflammatory demyelinating disorder of the white matter often preceded by viral infection or recent vaccination. It is commonly seen in children and young adults, where prognosis is favorable. To date, very few cases of ADEM has been reported in middle-aged and elderly patients. Approximately 25% of ADEM cases in adults will develop Multiple Sclerosis (MS) within five years of initial presentation but majority of individuals do not progress beyond three months. Stiff-Person Syndrome (SPS) is a rare neurologic disorder with wide variety of symptoms that mimic neuro-inflammatory and demyelinating disease which causes a delay in the diagnosis. Here we present a case of ADEM in a middle-aged adult masquerading as SPS on initial presentation.