Introduction: The Cognitive Debt hypothesis proposes that repetitive negative thinking (RNT), a modifiable process common to many psychological risk factors for Alzheimer's disease (AD) may itself increase risk. We sought to empirically examine relationships between RNT and markers of AD, compared with anxiety and depression symptoms. Methods: Two hundred and ninety-two older adults with longitudinal cognitive assessments, including 113 with amyloid-positron emission tomography (PET) and tau-PET scans, from the PREVENT-AD cohort and 68 adults with amyloid-PET scans from the IMAP+ cohort were included. All participants completed RNT, anxiety, and depression questionnaires. Results: RNT was associated with decline in global cognition (P = .02); immediate (P = .03) and delayed memory (P = .04); and global amyloid (PREVENT-AD: P = .01; IMAP+: P = .03) and entorhinal tau (P = .02) deposition. Relationships remained after adjusting for potential confounders. Discussion: RNT was associated with decline in cognitive domains affected early in AD and with neuroimaging AD biomarkers. Future research could investigate whether modifying RNT reduces AD risk.