OBJECTIVES: Social isolation among older adults is an important but under-recognized risk for poor health outcomes. Methods are needed to identify subgroups of older adults at risk for social isolation. METHODS: We constructed a typology of social isolation using data from the National Health and Aging Trends Study (NHATS) and estimated the prevalence and correlates of social isolation among community-dwelling older adults. The typology was formed from four domains: living arrangement, core discussion network size, religious attendance, and social participation. RESULTS: In 2011, 24% of self-responding, community-dwelling older adults (65+ years), approximately 7.7 million people, were characterized as socially isolated, including 1.3 million (4%) who were characterized as severely socially isolated. Multinomial multivariable logistic regression indicated that being unmarried, male, having low education, and low income were all independently associated with social isolation. Black and Hispanic older adults had lower odds of social isolation compared with white older adults, after adjusting for covariates. DISCUSSION: Social isolation is an important and potentially modifiable risk that affects a significant proportion of the older adult population.