Background: and Purpose Neuropsychiatric symptoms (NPS) are frequent in dementia and mild cognitive impairment (MCI), yet their reported associations with white matter hyperintensities (WMHs) remain inconsistent. We examined item-level associations between WMHs and NPS across Alzheimer’s disease dementia (ADD), dementia with Lewy bodies (DLB), and amnestic MCI (aMCI). Methods: This multicenter retrospective study involved 523 patients: 276 with ADD, 67 with DLB, and 180 with aMCI. Three-dimensional T1-weighted and T2–fluid-attenuated inversion recovery magnetic resonance images were processed with BAAD (Brain Anatomical Analysis using Diffeomorphic Deformation) software to quantify WMH volume (WMHV) and gray matter volume (GMV), normalized to total intracranial volume (TIV). NPS were assessed using the Neuropsychiatric Inventory. Multiple regression analyses were performed within each group, adjusting for GMV/TIV, age, sex, years of education, Clinical Dementia Rating–Sum of Boxes score, and institution. Results: In the aMCI group, higher WMHV/TIV was positively associated with depression, apathy, and disinhibition, and negatively with delusions. Subregional analyses revealed NPSspecific topographic patterns. No significant associations were detected in the ADD or DLB group. GMV/TIV was positively associated with delusions in the ADD group, but negatively associated with hallucinations in the DLB group. Conclusions Associations between WMHs and NPS were evident in the aMCI group but absent in the ADD and DLB groups, suggesting a stage-dependent effect. WMHs may contribute to NPS primarily in prodromal stages, whereas their influence may be overshadowed by neurodegenerative pathology in dementia. Clarifying the mechanisms underlying WMHs is critical for evaluating their potential as intervention targets.