Sexualized gender stereotypes (SGS) include the belief that girls should singularly prioritize their sexualized attractiveness for the attention and approval of boys. By elementary school, boys and girls perceive girls' sexualized attractiveness to be incompatible with intelligence and competence. In the current 2-year study, we examined whether girls' higher SGS endorsement in seventh grade predicted a diminished mastery goal orientation and lower perceptions of academic ability in eighth grade and whether this was moderated by gender typicality and self-monitoring. Cross-lagged panel analyses tested whether earlier academic attitudes better predicted later SGS endorsement than the inverse. The study included 77 girls in the final sample from four public middle schools (M[subscript ageT1] = 12.4, SD = 0.57). The sample was ethnically diverse (45% identified as White, 21% as Latinx, 19% as Black/African American, and 14% as multiracial). Girls' greater endorsement of SGS in the seventh grade predicted lower academic self-efficacy later, controlling for age, academic ability, and earlier levels of academic attitudes. Highlighting a likely feedback loop, earlier academic self-efficacy equally predicted later SGS endorsement. For highly gender-typical girls, greater SGS endorsement also predicted lower mastery goal orientation over time.