BACKGROUND: Alzheimer's disease (AD) is characterized by the abnormal accumulation of amyloid-beta (Aβ) and tau that can be quantified in vivo through cerebrospinal fluid (CSF) sampling. Physical activity has emerged as a possible modifier of AD risk; however, its impact on CSF biomarkers and cognitive function is not yet fully understood. We examined whether higher levels of physical activity modifies associations between AD CSF biomarkers and cognitive function. METHODS: One hundred and seventeen adults free of dementia from the BIOCARD study (mean age 72.2 ± 8.0 years, 70% women) wore a wrist accelerometer for 1 week, underwent lumbar puncture to collect CSF, and completed a comprehensive neuropsychological exam. Multivariable linear regression analyses were used to examine whether physical activity (total activity counts over the 10 most active hours of the day) moderates the association between AD CSF biomarkers [Aβ42/40, phosphorylated tau (p-tau181), and total tau] and cognitive composite scores (episodic memory, executive function). RESULTS: There were significant interactions between physical activity and p-tau181 ( = 0.016) as well as between physical activity and total tau ( = 0.004) in relation to the executive function composite score. Among participants with higher levels of physical activity, the adverse relationship between CSF-measured tau and executive function was diminished. In contrast, there were no significant interactions for episodic memory, and physical activity did not interact with Aβ42/40 (all interactions > 0.05). CONCLUSION: A physically active lifestyle may provide protection against AD-related cognitive decline by reducing the impact of tau pathology. HIGHLIGHTS: Older age was associated with lower levels of physical activity, worse CSF biomarker profiles, and poorer cognition.Physical activity moderates the impact of tau pathology on executive function but shows no significant effect on amyloid-beta pathology.Physical activity may enhance cognitive reserve, thereby attenuating the influence of accumulating AD pathology on cognition.