This study aims to investigate how teachers' trust in their students relates to reading comprehension achievement in socially and ethnically segregated elementary schools in Flanders (Belgium) by taking into account class composition characteristics. It is examined how student variables, ethnic diversity and the proportion of non-native students in the class, and teachers' trust in their students relate to reading comprehension achievement and learning growth. A 3-level multivariate repeated measures analysis was conducted. At 2 measurement occasions, reading tests and questionnaires were administered to a sample (n = 417) of 7- and 8-year-old students in 32 classes. Teachers' trust in their students was found to be a key factor relating to learning growth in reading comprehension, and mediated the relationship between the level of ethnic diversity in the class and learning growth. Teachers with a higher level of trust in their students seem to foster more learning growth in reading comprehension.