Objective: We sought to identify motor features that would allow the delineation of individuals with sleep study-confirmed idiopathic rapid eye movement sleep behaviour disorder (iRBD) from controls and Parkinson’s disease (PD) using a customised smartphone application. Methods: 334 PD, 104 iRBD, and 84 control participants performed seven tasks to evaluate voice, balance, gait, finger tapping, reaction time, rest tremor, and postural tremor. Smartphone recordings were collected both in clinic and at home under non-controlled conditions over several days. All participants underwent detailed parallel in-clinic assessments. Using only the smartphone sensor recordings, we sought to: (1) discriminate whether the participant had iRBD or PD and, (2) identify which of the above seven motor tasks were most salient in distinguishing groups. Results: Statistically significant differences based on these seven tasks were observed between the three groups. For the three pairwise discriminatory comparisons: (1) Controls versus iRBD, (2) Controls versus PD, and (3) iRBD versus PD, mean sensitivity and specificity values ranged from 84.6% to 91.9%. Postural tremor, rest tremor, and voice were the most discriminatory tasks overall, while reaction time was least discriminatory. Conclusions: Prodromal forms of PD include the sleep disorder iRBD, where subtle motor impairment can be detected using clinician based rating scales (e.g UPDRS), which may lack the sensitivity to detect and track granular change. Consumer grade smartphones can be used not only to accurately separate iRBD from controls, but also iRBD from PD participants, providing growing consensus for the utility of digital biomarkers in early and prodromal PD.