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 2025