According to expectancy-value theory, the gender stereotypes of significant others such as parents, peers, or teachers affect students’ competence beliefs, values, and achievement-related behavior. Stereotypically, gender beliefs about reading favor girls. The aim of this study was to investigate whether teachers’ gender stereotypes in relation to reading—their belief that girls outperform boys—have a negative effect on the reading self-concept of boys, but not girls. We drew on a longitudinal study comprising two occasions of data collection: toward the beginning of Grade 5 (T1) and in the second half of Grade 6 (T2). Our sample consisted of 54 teachers and 1,358 students. Using multilevel modeling, controlling for T1 reading self-concept, reading achievement, and school track, we found a negative association between teachers’ gender stereotype at T1 and boys’ reading self-concept at T2, as expected. For girls, this association did not yield a significant result. Thus, our results provide empirical support for the idea that gender differences in self-concept may be due to the stereotypical beliefs of teachers as significant others. In concluding, we discuss what teachers can do to counteract the effects of their own gender stereotypes.