Objectives. To evaluate the association between living alone and suicide and how it varies across sociodemographic characteristics. Methods. A nationally representative sample of adults from the 2008 American Community Survey (n = 3 310 000) was followed through 2019 for mortality. Cox models estimated hazard ratios of suicide across living arrangements (living alone or with others) at the time of the survey. Total and sociodemographically stratified models compared hazards of suicide of people living alone to people living with others. Results. Annual suicide rates per 100 000 person-years were 23.0 among adults living alone and 13.2 among adults living with others. The age-, sex-, and race/ethnicity-adjusted hazard ratio of suicide for living alone was 1.75 (95% confidence interval = 1.64, 1.87). Adjusted hazards of suicide associated with living alone varied across sociodemographic groups and were highest for adults with 4-year college degrees and annual incomes greater than $125 000 and lowest for Black individuals. Conclusions. Living alone is a risk marker for suicide with the strongest associations for adults with the highest levels of income and education. Because these associations were not controlled for psychiatric disorders, they should be interpreted as noncausal. (Am J Public Health. 2022;112(12):1774-1782. https://doi.org/10.2105/AJPH.2022.307080).