Introduction: Non-invasive clinical tests that enable early detection of Parkinson’s disease (PD) can alter treatment planning and the course of the disease. Gait impairments are among the most common and debilitating symptoms of late-stage PD, inspiring significant research into gait-based biomarkers of PD; however, the gait analysis approaches used at the clinical point-of-care lack the accuracy needed to differentiate PD-related gait deficits from those that naturally occur with aging. Inspired by the neuroscience of auditory-motor entrainment, we present an auditory-motor probe of PD that uses a targeted assessment of gait entrainment across a range of personalized auditory rhythms to detect PD.  Methods: Thirty-one individuals with mild-to-moderate PD (PwPD; UPDRS=25) and 32 healthy controls (n=12, 18-30 years; n=20, 65-80 years) completed two personalized rhythm sweeps with gait entrainment quantified by a thigh-mounted inertial sensor system and custom analysis algorithms.  Results: Individuals with mild-to-moderate PD did not exhibit differences in common spatiotemporal gait metrics (speed, stride length, cadence, stride time variability) compared to healthy individuals (p = 0.10 - 0.82). In contrast, PwPD demonstrated reduced auditory-motor entrainment compared to healthy controls (p = 0.001). When using a 7.7% entrainment cutoff, the auditory-motor probe achieved good diagnostic accuracy in identifying PwPD, substantially outperforming spatiotemporal gait metrics that showed limited diagnostic accuracy (AUC=0.73 vs 0.59). In the use case of a diagnostic screening tool, a lower entrainment cutoff of 6.2% markedly enhances sensitivity (0.87) without substantially compromising overall performance.  Conclusion: Targeted measurements of auditory-motor entrainment can be used as a sensitive and non-invasive diagnostic biomarker of mild-to-moderate PD that outperforms conventional gait analysis. Further development of this auditory-motor probe as an early diagnostic tool is warranted.