Math and science motivational beliefs are essential in understanding students' science, technology, engineering, and math (STEM) achievement and choices in high school and college. Drawing on the Eccles' expectancy-value theory and Arnett's emerging adulthood framework, this study examined the relations among high school students' motivational beliefs in ninth grade and their STEM course taking and grade point average (GPA) throughout high school as well as their STEM major choice in college. In addition, we examined subgroup differences across (a) gender and (b) college generation status by testing mean-level differences as well as whether these relations between math and science motivational beliefs and STEM outcomes varied by gender and college generation status. Using nationally representative data from the High School Longitudinal Study (N = 14,040; M[subscript age] = 14; 51% female students), this study found that adolescents' math and science motivational beliefs at the beginning of high school were positively associated with STEM achievement and course taking throughout high school and college major choices 7 years later. The results showed that female and first-generation college students had lower math and science self-concept of ability and were less likely to pursue a STEM major in college. However, in most cases, the relations among indicators did not vary by gender and college generation status. This study provided insights for policymakers and practitioners that gender and college generation gaps in STEM are evident at least by the beginning of high school and carry forward to their STEM college choices.