Alzheimer's disease (AD) is the most prevalent age-related dementia and causes memory, reasoning, and social skills to deteriorate. In recent years many studies have explored the genetic risk of AD, but less work has been done to identify a brain imaging-based AD risk measure. The current study proposed a new neuroimaging-based measure of AD risk, called brain-wide risk score or BRS, based on multimodal brain features. Using the proposed AD BRS, we identified four AD biotypes from a large sample of subjects (N>37,000) from the UK Biobank dataset: one with high AD BRS, one with low AD BRS, and two with moderate AD BRS. Next, we further showed that the cognitive scores of the biotype with lower AD BRS are significantly better than those of other biotypes.