OBJECTIVE: Our objective was to investigate whether type 2 diabetes mellitus (T2DM) influences neurodegeneration in a manner similar to Alzheimer disease (AD), by promoting brain beta-amyloid (Abeta) or tau. METHODS: We studied the cross-sectional associations of T2DM with cortical thickness, brain Abeta load, and CSF levels of Abeta and tau in a sample of people from the Alzheimer s Disease Neuroimaging Initiative with diagnoses of AD dementia, mild cognitive impairment, and normal cognition. All (n=816) received MRI, and a subsample underwent brain amyloid imaging (n=102) and CSF Abeta and tau measurements (n=415). Analyses were performed across and within cognitive diagnostic strata. RESULTS: There were 124 people with T2DM (mean age 75.5 years) and 692 without T2DM (mean age 74.1 years). After adjusting for age, sex, total intracranial volume, APO epsilon4 status, and cognitive diagnosis, T2DM was associated with lower bilateral frontal and parietal cortical thickness (mL) (beta=-0.03, p=0.01). T2DM was not associated with 11C Pittsburgh compound B standardized uptake value ratio (AU) in any brain region or with CSF Abeta42 levels (pg/mL). T2DM was associated with greater CSF total tau (pg/mL) (beta=16.06, p=0.04) and phosphorylated tau (beta=5.84, p=0.02). The association between T2DM and cortical thickness was attenuated by 15 by the inclusion of phosphorylated tau. CONCLUSIONS: T2DM may promote neurodegeneration independent of AD dementia diagnosis, and its effect may be driven by tau phosphorylation. The mechanisms through which T2DM may promote tau phosphorylation deserve further study.