Finding novel biomarkers for Parkinson’s disease (PD) is crucial for early disease diagnosis, severity assessment and identifying novel disease-modifying drug targets. Our study aimed at investigating theGATA3mRNA levels in whole blood samples of idiopathic PD (iPD) patients with different disease severities as a biomarker for iPD. The present study is a cross-sectional, case-control study, with samples obtained from the Luxembourg Parkinson’s cohort (LuxPARK). iPD (N = 319) patients, along with age-matched controls without PD (non-PD; N = 319) were included in this study. BloodGATA3mRNA expression was measured using quantitative reverse transcription PCR (RT-qPCR) assays. The capacity ofGATA3expression levels to establish the diagnosis of iPD (primary end-point) and assess disease severity (secondary end-point) was determined. The blood levels ofGATA3were significantly lower in iPD patients, compared to non-PD controls (p≤ 0.001). Logistic regression models showed a significant association ofGATA3expression with iPD diagnosis after adjustment for the confounders (p= 0.005). Moreover, the addition ofGATA3expression to a baseline clinical model improved its iPD diagnosis capacity (p= 0.005). There was a significant association ofGATA3expression levels with the overall disease severity (p= 0.002), non-motor experiences of daily living (nm-EDL;p= 0.003) and sleep disturbances (p= 0.01). Our results suggest thatGATA3expression measured in blood may serve as a novel biomarker and may help in the diagnosis of iPD and assessment of disease severity.