BACKGROUND AND OBJECTIVE: To examine whether early weight change is associated with subsequent deterioration in cognitive function, including overall performance and specific domains, in Parkinson disease (PD). METHODS: This observational study used data from the Parkinson Progression Markers Initiative cohort. The patients underwent annual nonmotor assessments covering neuropsychiatric, sleep-related, and autonomic symptoms for up to 8 years of follow-up. Cognitive function was measured using the Montreal Cognitive Assessment (MoCA) and detailed neuropsychological testing. Linear mixed-effects models were applied to investigate the association of early weight change with longitudinal evolution of cognitive and other nonmotor symptoms. RESULTS: A total of 358 patients with early PD were classified into weight loss (decrease of >3% body weight during the first year; n = 98), weight maintenance (within ±3%; n = 201), and weight gain (increase of >3%; n = 59) groups. The weight loss group showed a significantly faster decline in MoCA scores than the weight maintenance group (ß = -0.19, 95% CI -0.28 to -0.10). With respect to specific cognitive domains, the weight loss group showed a steeper decline in sematic fluency test scores (ß = -0.37, 95% CI -0.66 to -0.08) and MoCA phonemic fluency scores (ß = -0.18, 95% CI -0.31 to -0.05) and, to a lesser extent, Letter-Number Sequencing scores (ß = -0.07, 95% CI -0.14 to 0.01) compared with the weight maintenance group. Conversely, the weight gain group showed a slower decline in the Symbol Digit Modalities Test scores (ß = 0.34, 95% CI 0.05 to 0.63), although no association was found with longitudinal changes in MoCA scores. We did not find any significant effects of weight change on the progression of other nonmotor symptoms. DISCUSSION: Early weight loss was associated with a faster progression of decline in global cognitive function and executive function in patients with PD, whereas early weight gain was associated with a slower progression of decline in processing speed and attention. The impact of early weight change on nonmotor symptoms seemed to be specific to cognition.