REM sleep behavior disorder (RBD) is so far a possibly underestimated yet well-described sleep disorder. Its major impact is the vigorous sleep behavior that often results in injuries to the patient himself or to people sleeping nearby. We treated a 64-year-old male with a clinically and polysomnographically confirmed diagnosis of RBD with 3 mg melatonin, which led to a significant reduction of motor activity during sleep, as measured by actigraphy (p < 0.0001 in all analyzed movement parameters), and a full clinical recovery over a 5-month treatment period. RBD phenomena gradually returned after melatonin administration was stopped. After 2 months' treatment, polysomnography showed no major changes except an increase of REM sleep (13 vs. 17% of sleep period time) and a better preservation of REM-sleep-associated muscle atonia. Our results suggest that melatonin might be able to reinforce REM sleep in RBD patients by enhancing its active inhibition of motor activity.