We report a case of rapid eye movement (REM) sleep behavior disorder (RBD) complicated by obstructive sleep apnea–hypopnea syndrome (OSAHS). The patient was admitted to our clinic complaining of apnea and snoring during sleep. Questioning revealed an abnormal behavior and talking during sleep. A daytime polysomnography (D-PSG) confirmed both RBD and OSAHS. We tried clonazepam (CZP) even though the sedating side-effect might worsen OSAHS. Soon after starting treatment, all abnormal behaviors and subjective symptoms disappeared and oxygen saturation during sleep also improved.