Parkinson's disease (PD) is a common neurodegenerative disease, characterized clinically by both motor and non-motor symptoms. Pathologically, PD is hallmarked by the loss of dopaminergic neurons in the substantia nigra (SN) and the formation of α-synuclein (α-syn) containing inclusion bodies (Lewy pathology) in the surviving neurons. Diagnosis of PD is still based on clinical features. However, owing to the complexity, het-erogeneity, and overlapping of its symptoms with other Parkinsonian disorders, correct diagnosis of PD remains a challenge, especially in the early stages. Therefore, there is an urgent need for biomarkers that can help cor-rectly diagnose PD, differentiate PD from other Parkin-sonian disorders, monitor the progression of the disease, and evaluate the therapeutic efficacy. Various molecules have been investigated for their utility in diagnosing PD, among which α-syn is the most extensively investigated one due to its close implication in the etiology and patho-genesis of PD and related diseases. During the past de-cade, various species of α-syn, including total, oligomer-ic, and phosphorylated α-syn in various tissues, have been investigated for their utility as a potential biomarker for PD diagnosis and differential diagnosis. Various forms of α-syn in body fluids, including cerebrospinal fluid (CSF), blood plasma, and saliva, are among the ones that are ex-tensively investigated, since the body fluids are relatively accessible compared to the peripheral tissues. The aim of this review is to summarize the progress of studies on the utility of α-syn in body fluid as a biomarker for PD diag-nosis and differential diagnosis.