Maintaining adolescents' engagement with STEM (science, technology, engineering, and math) in and out of school may help ensure that adolescents are prepared to enter the STEM workforce. This study aims to extend prior work by documenting internal and external factors that matter for both STEM class engagement as well as engagement with STEM outside of school through STEM activism. Participants included ninth and tenth grade students (N = 852) from ethnically diverse public schools in the Southeastern United States, approximately evenly divided by gender. Findings from regression analyses revealed that girls and participants who perceive educational barriers to STEM were less engaged in STEM classes, whereas those who reported learning about more male scientists in class, and those who reported higher levels of belonging, STEM growth mindset, and STEM motivation were more engaged in STEM classes. Those who reported higher critical motivation, critical action, belonging, and STEM motivation were more engaged in STEM activism outside of school. Findings suggest that STEM teachers and out-of-school program developers may learn new ways to engage students from each other. Further, findings highlight some factors that may promote engagement in STEM both in and out of schools such as belonging and STEM motivation.