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The 2021 World Health Organization (WHO) grading system of isocitrate dehydrogenase (IDH)‐mutant astrocytomas relies on histological features and the presence of homozygous deletion of the cyclin‐dependent kinase inhibitor 2A and 2B (CDKN2A/B). DNA methylation profiling has become highly relevant in the diagnosis of central nervous system (CNS) tumors including gliomas, and it has been incorporated into routine clinical diagnostics in some countries. In this study, we, therefore, examined the value of DNA methylation‐based classification for prognostication of patients with IDH‐mutant astrocytomas. We analyzed histopathological diagnoses, genome‐wide DNA methylation array data, and chromosomal copy number alteration profiles from a cohort of 385 adult‐type IDH‐mutant astrocytomas, including a local cohort of 127 cases and 258 cases from public repositories. Prognosis based on WHO 2021 CNS criteria (histological grade and CDKN2A/B homozygous deletion status), other relevant chromosomal/gene alterations in IDH‐mutant astrocytomas and DNA methylation‐based subclassification according to the molecular neuropathology classifier were assessed. We demonstrate that DNA methylation‐based classification of IDH‐mutant astrocytomas can be used to predict outcome of the patients equally well as WHO 2021 CNS criteria. In addition, methylation‐based subclassification enabled the identification of IDH‐mutant astrocytoma patients with poor survival among patients with grade 3 tumors and patients with grade 4 tumors with a more favorable outcome. In conclusion, DNA methylation‐based subclassification adds prognostic information for IDH‐mutant astrocytomas that can further refine the current WHO 2021 grading scheme for these patients. [ABSTRACT FROM AUTHOR]