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Background: Cognitive deficits are associated with poor quality of life and increased risk of development of dementia in patients with Parkinson's disease (PD) psychosis. The trajectory of cognitive decline in PD psychosis remains however unclear., Objective: We examined this using data from the Parkinson's Progression Markers Initiative study., Methods: We analysed data from patients with drug-naïve PD (n=676) and healthy controls (HC, n=187) over 5 years, and examined all cognitive measures assessed at each time point. We classified patients with PD into those who developed psychosis over the course of the study (PDP) and those without psychosis throughout (PDnP) using the Movement Disorders Society Unified Parkinson's Disease Rating Scale part I hallucinations/psychosis item. We used linear mixed-effect models with restricted maximum likelihood. Age, sex, ethnicity, education and neuropsychiatric and PD-specific symptoms were entered as covariates of interest., Findings: There were no baseline cognitive differences between PD patient groups. There were differences in cognitive performance between PD and HC across the majority of the assessments.Patients with PDP exhibited greater cognitive decline over 5 years compared with PDnP across most domains even after controlling for sociodemographics, depression, sleepiness, rapid eye movement sleep behaviour disorder and motor symptom severity (immediate recall, b=-0.288, p = 0.003; delayed recall, b=-0.146, p = 0.003; global cognition, Montreal Cognitive Assessment, b=-0.206, p < 0.001; visuospatial, b=-0.178, p = 0.012; semantic fluency, b=-0.704, p = 0.002; processing speed, b=-0.337, p = 0.029)., Conclusions: Patients with PD psychosis exhibited decline in semantic aspects of language, processing speed, global cognition, visuospatial abilities and memory, regardless of sociodemographic characteristics, neuropsychiatric and motor symptoms. These cognitive domains, particularly semantic aspects of language may therefore play an important role in PD psychosis and warrant further investigation., Trial Registration Number: NCT01141023., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2024. Re-use permitted under CC BY-NC. Published by BMJ.)