INTRODUCTION: Alzheimer's disease (AD) in Down syndrome (DS) is associated with changes in brain structure. It is unknown if thickness and volumetric changes can identify AD stages and if they are similar to other genetic forms of AD. METHODS: Magnetic resonance imaging scans were collected for 178 DS adults (106 nonclinical, 45 preclinical, and 27 symptomatic). Cortical thickness and subcortical volumes were compared between DS groups and evaluated as a staging metric using receiver operating characteristic analyses. Thickness patterns were compared to those previously reported in autosomal-dominant AD (ADAD). RESULTS: Decreased parietal and temporal lobe thickness differentiated amyloid positivity (area under the curve [AUC] = 0.83) and impairment (AUC = 0.81), and slightly outperformed subcortical volumes (AUC = 0.8/0.74). Thickness differences in DS were more widespread, severe, and had better discriminative ability than ADAD. DISCUSSION: Cortical thickness can stage AD pathology in DS. Identification of brain regions affected by AD may aid in tracking disease course and evaluating treatment effects. HIGHLIGHTS: DSAD is associated with reduced temporal and parietal cortical thickness. DSAD is associated with smaller hippocampal and striatal volumes. Thickness differences can stage DSAD better than other forms of AD. DSAD thickness differences are more extensive and severe than ADAD.