Objective: To investigate the relationship between Alzheimer's disease biomarkers and neuropsychiatric symptoms. Methods: Data from two large cohort studies, the Dutch Parelsnoer Institute – Neurodegenerative Diseases and the Alzheimer's Disease Neuroimaging Initiative was used, including subjects with subjective cognitive decline (N = 650), mild cognitive impairment (N = 887), and Alzheimer's disease dementia (N = 626). Cerebrospinal fluid (CSF) levels of Aβ42, t-tau, p-tau, and hippocampal volume were associated with neuropsychiatric symptoms (measured with the Neuropsychiatric Inventory) using multiple logistic regression analyses. The effect of the Mini-Mental State Examination (as proxy for cognitive functioning) on these relationships was assessed with mediation analyses. Results: Alzheimer's disease biomarkers were not associated with depression, agitation, irritability, and sleep disturbances. Lower levels of CSF Aβ42, higher levels of t- and p-tau were associated with presence of anxiety. Lower levels of CSF Aβ42 and smaller hippocampal volumes were associated with presence of apathy. All associations were mediated by cognitive functioning. Conclusion: The association between Alzheimer's disease pathology and anxiety and apathy is partly due to impairment in cognitive functioning.