REM sleep behavior disorder (RBD) is a parasomnia which is characterized by a loss of the normal skeletal muscle atonia associated with REM sleep with dream enactment behavior that is generally violent in nature. Serious injury to the patient or the bed partner can result from RBD episodes. Neuroanatomical and physiological integrity of structures in the brainstem is important for the continuity of REM atonia. Brain anomalies and lesions that affect the brainstem may play a causal role in the pathophysiology of RBD. In this case, the presentation of a 73-year-old woman who is diagnosed with RBD accompanying arachnoid cyst in posterior fossa was made.