ABSTRACT As teachers in training must learn key professional competencies, student teachers often undergo training approaches that are either based in theory or practice. To more comprehensively promote student teachers’ professional competencies, we examined the effectiveness of a combined theory-practice-based professionalisation program on student teachers’ knowledge, self-efficacy, and instructional beliefs. The intervention, focusing on differentiated reading instruction, included theoretical training and a practical application in which student teachers implemented an evidence-based reading training in schools. The control group (N = 44) and the experimental group (N = 35) consisted of student teachers in their practical semester at university, while the experimental group additionally received the intervention. Compared to the control group, the experimental group experienced self-reported improvements in knowledge, self-efficacy, and teacher-centred beliefs, and these improvements had developed early in the training process. Overall, this study emphasises the effectiveness of the combined training approach and the importance of early theoretical training.