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Background and Aims: Neurodegeneration underpins the pathological processes of younger‐onset dementia (YOD) and has been implicated in primary psychiatric disorders (PSYs). Cerebrospinal fluid (CSF) neurofilament light (NfL) has been used to investigate neurodegeneration severity through correlation with structural brain changes in various conditions, but has seldom been evaluated in YOD and PSYs. Methods: This retrospective study included patients with YOD or PSYs with magnetic resonance imaging (MRI) of the brain and CSF NfL analysis. Findings from brain MRI were analysed using automated volumetry (volBrain) to measure white matter (WM), grey matter (GM) and whole brain (WB) volumes expressed as percentages of total intracranial volume. Correlations between NfL and brain volume measurements were computed whilst adjusting for age. Results: Seventy patients (47 with YOD and 23 with PSY) were identified. YOD types included Alzheimer disease and behavioural variant frontotemporal dementia. PSY included schizophrenia and major depressive disorder. MRI brain sequences were either fast spoiler gradient‐echo (FSPGR) or magnetization‐prepared rapid acquisition gradient‐echo (MPRAGE). In the total cohort, higher NfL was associated with reduced WB in the FSPGR and MPRAGE sequences (r = −0.402 [95% confidence interval (CI), −0.593 to −0.147], P = 0.008 and r = −0.625 [95% CI, −0.828 to −0.395], P < 0.001, respectively). Higher NfL was related to reduced GM in FSPGR (r = 0.385 [95% CI, −0.649 to −0.014], P = 0.017) and reduced WM in MPRAGE (r = −0.650 [95% CI, −0.777 to −0.307], P < 0.001). Similar relationships were seen in YOD, but not in PSY. Conclusion: Higher CSF NfL is related to brain atrophy in YOD, further supporting its use as a nonspecific marker of neurodegeneration severity. [ABSTRACT FROM AUTHOR]