OBJECTIVE: Assess the performance of serum phosphorylated tau 217 (p-tau217) and neurofilament light chain (NfL) in predicting risk of cognitive impairment or phenoconversion to dementia in individuals with iRBD. METHODS: We measured serum p-tau217 and NfL levels by electrochemiluminescence across 4 polysomnographically confirmed iRBD cohorts (n = 300), including individuals who phenoconverted to Parkinson's disease (PD) (n = 51), dementia with Lewy bodies (DLB) (n = 22), and multiple system atrophy (MSA) (n = 5). RESULTS: Serum p-tau217 levels were increased in individuals with iRBD and cognitive impairment (CI) on testing defined as Montreal Cognitive Assessment <26 or subthreshold parkinsonism. p-Tau217 differentiated individuals with iRBD who developed PD with CI (PD-CI) or DLB from PD phenoconverters with normal cognition (area under curve [AUC] = 0.82; 95% confidence interval, 0.70-0.93) and from iRBD non-phenoconverters with normal cognition (AUC = 0.83; 95% confidence interval, 0.77-0.89). NfL levels did not correlate with cognitive or motor scores and marginally improved p-tau217 performance (AUC = 0.85; 95% confidence interval, 0.78-0.92), but were notably elevated in iRBD individuals who phenoconverted to MSA. Individuals with p-tau217 in the top quartile were 8 times more likely to phenoconvert to PD-CI or DLB compared to the bottom quartile (hazard ratio = 8.30; 95% confidence interval, 2.49-27.65). INTERPRETATION: Serum p-tau217, but not NfL, is a useful biomarker of cognitive impairment in iRBD that could be integrated into a multimodal prognostic indicator when stratifying risk of phenoconversion. ANN NEUROL 2026;99:912-921.