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Introduction: We examined the relationship between sedentary behavior (SB), moderate-to-vigorous physical activity (MVPA), and white matter hyperintensity (WMH) volumes, a common magnetic resonance imaging (MRI) marker associated with risk of neurodegenerative disease in middle-aged to older adults., Methods: We used data from the UK Biobank ( n  = 14,415; 45 to 81 years) that included accelerometer-derived measures of SB and MVPA, and WMH volumes from MRI., Results: Both MVPA and SB were associated with WMH volumes (β MVPA  = -0.03 [-0.04, -0.01], p  < 0.001; β SB  = 0.02 [0.01, 0.03], p  = 0.007). There was a significant interaction between SB and MVPA on WMH volumes (β SB×MVPA  = -0.015 [-0.028, -0.001], p SB×MVPA  = 0.03) where SB was positively associated with WMHs at low MVPA, and MVPA was negatively associated with WMHs at high SB., Discussion: While this study cannot establish causality, the results highlight the potential importance of considering both MVPA and SB in strategies aimed at reducing the accumulation of WMH volumes in middle-aged to older adults., Highlights: SB is associated with greater WMH volumes and MVPA is associated with lower WMH volumes.Relationships between SB and WMH are strongest at low levels of MVPA.Associations between MVPA and WMH are strongest at high levels of SB.Considering both SB and MVPA may be effective strategies for reducing WMHs., Competing Interests: The authors declare no conflict of interest. Author disclosures are available in the Supporting information., (© 2024 The Author(s). Alzheimer's & Dementia: Diagnosis, Assessment & Disease Monitoring published by Wiley Periodicals LLC on behalf of Alzheimer's Association.)