Dermatomyositis (DM) is a rare connective tissue disease characterized by skin lesions and inflammatory changes observed in muscle biopsy findings. A definitive diagnosis of DM requires a characteristic rash in addition to proximal muscle weakness and muscle enzyme level elevation. DM is twice as common in women as men, with an age of onset of approximately 50 years. This case report describes a 29-year-old patient with low back pain and proximal muscle weakness in the legs diagnosed as lumbar disc herniation who was then referred by the neurosurgery department to our clinic. A physical examination revealed the characteristic skin lesions for dermatomyositis. Needle electromyography and a skin biopsy were performed, and corticosteroid treatment was initiated. In misdiagnosed patients, fatty infiltration in the muscles may cause irreversible weakness and gait disturbance. Early suppression of inflammation is important and can yield a dramatic response to treatment.