We describe a patient with frontotemporal dementia (FTD), a tauopathy, who also showed clinical and polysomnographic features of REM sleep behavior disorder (RBD). The patient is a 78-year-old male with a 1 year history of behavioral dysfunction involving emotion, character and social functioning. Brain imaging and the results of neuropsychological testing were consistent with a diagnosis of FTD. Sleep symptom onset occurred some years before the behavioral changes, and consisted of unpleasant dreams, vocalization, and prominent motor behaviors. A polysomnography confirmed the diagnosis of RBD. Our findings support the hypothesis that RBD, although more frequent in synucleinopathies, might be a pathological stage in the development of almost every neurodegenerative disorder in which the pathological process involves the cerebral structures that regulate muscle tone during REM sleep.