**Objective**: We examine the trajectories of and the dynamic interplay between cognitive functioning and depressive symptoms in patients with Parkinson's disease (PD) in comparison to healthy controls (HC) from an intraindividual perspective. **Method**: The DeNoPa study is a single-center, observational, longitudinal study with biennial follow-ups over 8 years. The present analyses are based on 123 PD (79 male) and 107 HC (64 male) with a mean age of 64.1. PD and HC completed a comprehensive battery of neurological tests and scales assessing depressive symptoms. To study their trajectories and the dynamic interplay we used a random-intercept cross-lagged panel model. **Results**: Cognitive abilities of PD were on average d = -0.67 worse at baseline and *d* = -1.22 at 8-years follow-up in comparison to HC. Depressive symptoms in PD showed a large variability and followed a U-shaped trajectory. From an intraindividual perspective, greater impairments in cognitive abilities were subsequently associated with increased depressive symptoms (*b* = -0.60, *p* = .03), whereas the effect in the opposite direction was not significant. **Conclusions**: We found indication that a decline on a global composite scale of cognition can be seen as precursor of depressive symptoms in patients with PD. To counter cognitive losses and the subsequent mood deterioration, patient education and early cognitive (and behavioral) enrichment seem promising candidates for treatment.
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