Assessment for learning (AfL) can facilitate students' development of metacognition. However, teachers often struggle with the implementation of AfL in their classroom. The dynamic approach can help teachers develop professionally in the complex competency that is AfL. The dynamic approach is based on four principles: (a) addressing the professional needs of teachers, (b) integrating skills, (c) explaining underlying mechanisms, and (d) implementing support and feedback. In this experimental study, we investigated the effect on students' metacognition of an AfL teacher professional development program for secondary mathematics teachers based on the dynamic approach (n[subscript teachers] = 47, n[subscript students] = 803). We found a statistically significant positive effect on students' ability to predict (d = 0.24) and evaluate (d = 0.22), but not on students' ability to plan.