Stiff-Person Syndrome (SPS) is a rare disorder that is characterized by stiffness and rigidity of the axial, abdominal, and extremity muscles. It is associated with several autoimmune diseases or paraneoplastic conditions. We report a case of SPS with Hashimoto's thyroiditis who complained of low back pain. Her physical examination was remarkable in that her abdominal and axial muscles were firm to palpation, and she had hyperlordosis of the spine. Active range of motion of the spine was limited, and she had a robotic gait. Electromyography demonstrated continuous motor unit activity in the agonist and antagonist muscles. Diagnosis of SPS was confirmed because antiglutamic acid decarboxylase was present in the cerebrospinal fluid. She favorably responded to intravenous administration of immunoglobulin, diazepam, and baclofen and stretching, strengthening, and range of motion exercises.