Cognitive symptoms are common in Parkinson's disease (PD), yet the underlying brain mechanisms remain poorly understood. To address this gap, we investigated the relationship between functional connectivity and cognition at multiple time points using longitudinal functional MRI (fMRI) and cognitive assessments from the Parkinson's Progression Marker Initiative (PPMI). We calculated resting-state functional connectivity within and between three key cortical brain networks that have been linked with cognitive function in PD: the frontoparietal network (FPN); the salience network (SAL); and the default mode network (DMN). Cognitive function was assessed with the Montreal Cognitive Assessment (MoCA). Linear mixed-effects modeling revealed that decreased FPN-DMN functional connectivity is associated with lower MoCA scores over time. This finding suggests that cortical connectivity is associated with and may contribute to the progression of cognitive symptoms in PD. Our findings advance knowledge about cognitive changes in PD and emphasize the importance of functional brain network architecture.