Drawing on the situated expectancy-value, dimensional comparison theories, and the intersectionality approach, this article examined the changes in adolescents' math and science motivational beliefs, the parental and college correlates of those beliefs, and the differences at the intersection of gender and college generation status (i.e., female and male first- and continuing-generation college students). Findings based on the nationally representative high-school longitudinal study data (N = 12,070; Mage = 14 years; 54% female students; 28% first-generation college students; and 14% Latinx, 9% Black, 10% Asian, and 57% White) suggest that although adolescents' math and science ability self-concepts declined during high school, their science interest remained stable, and their math and science utility values increased. Adolescents' motivational beliefs in ninth grade and the changes from ninth to 11th grade positively predicted whether they declared a science, technology, engineering, and mathematics (STEM) college major. Parents' ninth-grade STEM support was more consistently associated with adolescents' concurrent beliefs compared to the changes in their beliefs. Finally, we found that female first-generation college students, who were more likely to be Latinx and Black students, tended to have lower math and science motivational beliefs, received less parental STEM support, and were less likely to choose a STEM major than their peers. The findings of this study indicate adolescents' math and science motivational development in high school matters for their college majors and that certain understudied groups, including female first-generation college students, may experience acute marginalization in STEM and warrant further attention. (PsycInfo Database Record (c) 2024 APA, all rights reserved).