Purpose <p>Autonomic dysfunction, including early cardiovascular parasympathetic involvement, is sustained throughout Parkinson’s disease (PD). The vagus nerve, shown by ultrasonography, is atrophic in PD patients. We examined vagus nerve cross-sectional area (CSA) in controls versus PD patients, and its association with autonomic function in early PD.</p> Methods <p>We investigated 118 de novo PD patients and 29 controls. Vagus nerve CSA was measured using nerve ultrasonography at the bilateral mid-cervical area. Each patient was evaluated once by a randomly assigned sonographer; each control was measured twice by both. For accurate comparison, patients with diabetes were additionally excluded from the PD group. A 1:1 propensity score matching based on age, sex, hypertension, and dyslipidemia was performed. The CSA was compared between matched patients and controls. Clinical interviews, rating scales, and autonomic and olfactory function tests were performed. The association between these results and the CSA was analyzed.</p> Results <p>Median right and left vagus nerve CSA measured 1.3 and 1.2 mm², respectively, significantly lower than in controls (2.3 and 2.2 mm²). Partial correlation analysis showed a negative correlation between the right vagus CSA and abnormal heart rate response to deep breathing (rho = −0.191, <i>P</i> = 0.045).</p> Conclusion <p>Small right vagus nerve CSA is associated with early parasympathetic dysfunction in PD patients. These findings support the use of vagus nerve ultrasound as a potential non-invasive biomarker for identifying prodromal autonomic involvement and informing future early intervention research.</p>