Multiple sclerosis, a disorder of central nervous system demyelination, is a leading cause of disability in young people. Lesions of the spinal cord are usually less than two vertebral body segments long, peripherally located, and found in the cervical region. A 30-year-old woman had a 2-month history of back pain, urinary incontinence, and bilateral lower extremity weakness. Magnetic resonance imaging (MRI) of the spine showed an intramedullary spinal cord tumor from T4 to T8 with an intramedullary cyst from T1 to T4. After thoracic decompressive surgery, findings from biopsy of the cord lesion were consistent with multiple sclerosis. Postoperatively, the patient required an intensive rehabilitation program. This is the first reported case of histopathologically confirmed spinal cord demyelination presenting as an intramedullary thoracic cord tumor. Physiatrists should be alerted that demyelinating disease can mimic a spinal cord tumor, even on MRI, and must be considered in the differential diagnosis of a symptomatic spinal cord mass.