Success in science, technology, engineering, and mathematics (STEM) fields is often believed to require intellectual talent ("brilliance"). Given that many cultures associate men more than women with brilliance, this belief poses an obstacle to women's STEM pursuits. Here, we investigated the developmental roots of this phenomenon, focusing specifically on young children's beliefs about math (N = 174 U.S. students in Grades 1-4; 93 girls, 81 boys; 52% White, 17% Asian, 13% Hispanic/Latinx). We found that field-specific ability beliefs (FABs) that associate success in math (vs. reading/writing) with brilliance are already present in early elementary school. We also found that brilliance-oriented FABs about math are negatively associated with elementary school students' (and particularly girls') math motivation--specifically, their math self-efficacy and interest. The early emergence of brilliance-oriented FABs about math and the negative relation between FABs and math motivation underscore the need to understand the sources and long-term effects of these beliefs.