Domestic extremists pose a serious threat to the United States, with recent research suggesting that America is experiencing an upswing in radicalization. The present research explores an important motivational mechanism through which people could become an extremist: social isolation. When socially isolated from others, people are deprived of the certainty of normative structure, the fundamental need to belong, and meaningful social connection. In order to investigate the relationship between social isolation and extremism, we ran two cross-sectional studies with a student population (Study 1; N = 269) and a sample of U.S. residents (Study 2; N = 391). Results from Study 1 suggest that extremism is highest in individuals who spend more time alone and have a smaller social network. Additionally, this relationship is mediated by loneliness. Results from Study 2 suggest that greater loneliness, need to belong, and norm deviation is associated with greater extremism. Additionally, replicating the finding of the first study, the link between social isolation (operationalized as greater time spent alone and smaller social network) and extremism is mediated by loneliness. This research has important implications for understanding motivations that drive extremism and for countering violent extremism. The results of these studies are especially pertinent because people are more isolated now than ever before.