To ensure inclusion and equity in education, both teacher beliefs and practice are cited as influential. However, the connection between beliefs on student diversity and inclusive practice is often more complex than straightforward. Professional vision is theorised to be important in aligning beliefs with practice. Hence, this study investigates whether teachers' professional vision of differentiated instruction (DI) mediates between teachers' beliefs about teaching diverse learners and teachers' practice of DI, and is unique in investigating these constructs in concert with each other. Data are part of the Potential-project and were collected in a sample of secondary education teachers (N = 461) in Flemish schools (N = 23). Survey data measuring teachers' beliefs (i.e. growth mindset, professional beliefs about diversity and about differentiating the curriculum) and teachers' self-reported DI practice were combined with video-based comparative judgement data measuring teachers' professional vision of DI. Multilevel models show that both teachers with a more expert professional vision and less expert professional vision implement DI. For a more expert professional vision, beliefs about teaching diverse learners explain the association. For a less expert professional vision, teachers' reflective and experimental practice explains the relationship. Results suggest that aspects of teachers' competence are related in more complex ways than linear theoretical frameworks propose. Implications for teacher training and professionalisation are discussed.