Although research has shown that school context has consequences for intergroup attitudes, few studies have examined the role of teacher qualities, such as teacher support. In addition, previous research has paid limited attention to the mechanisms that could help to explain teacher effects. This 5-wave study (2010–2015) examined the effects of perceived teacher support on the anti-immigrant attitudes of Swedish majority youth (N = 671, Mage = 13.41, 50.2% girls, 34 classrooms). It also tested whether social trust would mediate these effects. The results of multilevel analyses showed that perceived teacher support was associated with less prejudice at all levels of analysis. At the within-person level, fluctuations in teacher support were related to fluctuations in youth prejudice: in years when, on average, adolescents perceived their teachers as more supportive, they reported lower prejudice. At the between-person level, adolescents who perceived their teachers as more supportive compared to their peers reported lower prejudice. Similarly, classrooms where students shared an experience of teacher support were lower in prejudice than classrooms with weaker teacher support. The results also showed that social trust explained teacher effects: adolescents who experienced their teachers as more supportive displayed higher levels of trust and, in turn, lower levels of prejudice than youth with less supportive teachers. These findings suggest that teachers can counteract the development of prejudice and facilitate social trust in adolescents by being supportive of them.