This study examined the role of teacher expectations in the emerging gender gaps in reading and mathematics in the first year of schooling. Therefore, we first investigated whether boys and girls differ in their vulnerability to teacher expectancy effects. Second, we analysed whether gender-specific effects of teacher expectations contribute to gender achievement gaps. Our analyses were based on 1,025 first-grade students in Germany. Among the majority of the students, boys and girls did not differ in their vulnerability to teacher expectancy effects. Further analyses examined a subgroup of students who were targets of relatively strong teacher expectation bias and who showed unexpectedly high or low achievement gains. In this specific subgroup, girls' mathematics achievement was more adversely affected by negatively biased expectations and benefited less from positive bias than boys' achievement. Mediation analyses revealed that teacher expectation bias did not substantially contribute to gender gaps in reading or mathematics.