Clinical isolated REM Sleep Behavior Disorder (iRBD) is a prodromal stage of α-synucleinopathies such as Parkinson's disease (PD). Longitudinal changes of gait as a marker of phenoconversion in iRBD have not been systematically evaluated yet. Therefore, twenty-one iRBD patients, nineteen matched healthy controls and fourteen PD patients (H&Y stage I - III) were assessed at baseline using a pressure-sensor walkway (GAITRite). While normalized gait velocity and walking endurance were significantly decreased in iRBD patients compared to controls, gait rhythm parameters such as single support further discriminated PD from iRBD and controls. IRBD patients at imminent risk of phenoconversion within the next 3.7 ± 0.6 years were distinguishable from non-converters by gait rhythm parameters such as single support. Since gait evaluation of our iRBD cohort showed promising results in predicting imminent phenoconversion, future multicenter trials should include gait analysis as an objective assessment of motor function to confirm our results.