Women and ethnic minoritized individuals are underrepresented in science, technology, engineering, and mathematics (STEM) domains in postsecondary education and in the workforce. The aim of the current study was to examine whether adolescents' perceptions of inclusivity, belonging, and discrimination in high school STEM classes are related to their STEM class engagement in and outside of school. In this study, ethnically diverse 9th–12th grade high school students from low-income public schools in the United States (N = 523, Mage = 15.72, SD = 1.24, 49.4% female) completed measures of classroom inclusivity, perceived teacher discrimination, belonging, STEM classroom engagement, and STEM activism orientation. Path analyses revealed direct effects of inclusion and perceived discrimination on STEM activism orientation. Further, findings demonstrated direct effects of inclusion on belonging and on belonging and both STEM classroom engagement and STEM activism orientation. Finally, findings revealed a significant indirect effect of inclusion on STEM classroom engagement through belonging.