As stereotype threat was initially examined in experimental settings, the effects of such threats have often been tested by temporarily manipulating social identity threats. This study expands the literature by examining 9th-grade adolescents’ naturalistic stereotype threat, using data from the National Study of Learning Mindsets in the United States (n ~= 6040, age: 13–17, Mage = 14.31, 6.9% Black boys, 6.5% Black girls, 13.1% Latinos, 12.3% Latinas, 31.5% White boys, 29.7% White girls). The results indicate that Black and Latinx students experience higher levels of stereotype threat in high school mathematics classrooms than do their White peers. When students perceive that their teachers have created fixed mindset climates, they experience greater stereotype threat. Stereotype threat, in turn, negatively predicts Black and Latino boys and White girls’ later achievement via anxiety. These findings highlight the importance of creating mathematics classrooms that cultivate a growth mindset and minimize social identity threat.