Increasing ethnic and racial diversity often fuels feelings of threat among ethnic-racial majorities (e.g., self-identified white Americans and European nationals). We contend that these threat perceptions depend on the policy context. Across four studies, we test whether more inclusive immigrant integration policies attenuate ethnic-racial majorities' threat reactions. Studies 1 to 3 (n = 469, 733, and 1745, respectively) used experimental methods with white American participants in the United States. Study 4 (n = 499,075) used secondary analysis of survey data comparing attitudes of nationals in 30 European countries and measured the impact of actual changes in diversity and policies over 10 years. Our results show that integration policies shape threat reactions even in those situations when increasing diversity could be seen as the most threatening: when narratives highlight the majority's impending minority position or when diversity suddenly increases. When policies are more inclusive toward immigrants, ethnic-racial majority participants report less threat (or no threat) in response to increasing diversity.