Drawing on Branscombe et al.’s conceptualization of social identity threat, we developed and validated a multidimensional measure of this concept: the Social Identity Threat Scale (SITS). We conducted three studies that examined social identity threat to U.S. Americans (Study 1, N = 468; Study 2, N = 457) and e-cigarette users (Study 3, N = 395) when they read negative messages directed at their social groups. Results provided evidence for the reliability, criterion-related validity, and construct validity of the SITS. Moreover, findings showed that social identity threat can be represented by five types of threat—categorization, distinctiveness, competence, morality, and generalized threats. The measure can be an essential tool for future research on social identity threat and for studies of intergroup relations more broadly.