Idiopathic rapid eye movement sleep behavior disorder (iRBD) likely represents the prodromal stage of synucleinopathy. The present study investigated how levels of serum uric acid (UA) affect cognition and motor function in patients with iRBD. A total of 42 patients with iRBD and 45 healthy controls were included. All participants were given cognitive tests and motor assessments. Serum UA concentrations were measured. The patients were further divided into two groups (high or low UA) according to serum UA level. The level of serum UA was similar between the patients with iRBD and the healthy controls, whereas the patients showed impaired executive, memory, and visuospatial functions. The patients with low UA levels had longer durations of RBD. Lower scores involving attention, executive function, and language domain were also found in the patients with low UA, whereas the scores of the patients with high UA were similar to those of the healthy controls. Regarding memory domain, the low UA group had worse scores than the healthy controls, whereas the scores of high UA group fell between those of the low UA group and the healthy controls. Motor function was not affected in any of the groups. UA affects cognitive function but not motor function in patients with iRBD, which could contribute to its antioxidant and neuroprotective roles.