Importance Resting-state functional connectivitymagnetic resonance imaging has great potential for characterizing pathophysiological changes during the preclinical phase of Alzheimer disease. Objective To assess the relationship between default mode network integrity and cerebrospinal fluid biomarkers of Alzheimer disease pathology in cognitively normal older individuals. Design, Setting, and Participants Cross-sectional cohort study at The Charles F. and Joanne Knight Alzheimer's Disease Research Center atWashington University in St Louis, St Louis, Missouri, among 207 older adults with normal cognition (Clinical Dementia Rating, 0). Main Outcomes and Measures Resting-state functional connectivitymagnetic resonance imaging measures of default mode network integrity. Results Decreased cerebrospinal fluid Aβ42 and increased cerebrospinal fluid phosphorylated tau181 were independently associated with reduced default mode network integrity, with the most prominent decreases in functional connectivity observed between the posterior cingulate and medial temporal regions. Observed reductions in functional connectivity were unattributable to age or structural atrophy in the posterior cingulate and medial temporal areas. Similar resting-state functional connectivitymagnetic resonance imaging findings in relation to cerebrospinal fluid biomarkers were obtained using region-of-interest analyses and voxelwise correlation mapping. Conclusions and Relevance Both Aβ and tau pathology affect default mode network integrity before clinical onset of Alzheimer disease.