Idiopathic rapid eye movement (REM) sleep behavior disorder is recognized as a prodromal stage of α-synucleinopathies such as Parkinson′s disease, Lewy body dementia and multiple system atrophy. It is important to timely identify early predictors that can predict early conversion into α-synucleinopathies. This review provided an update on classic and novel early predictors of α-synucleinopathies in patients with idiopathic REM sleep behavior disorder and provided a comprehensive understanding on the phenotypic transformation of the disease.