Background: Formative assessment has been reported to improve learners' learning in affluent contexts. However, very few studies have reported the impact of formative assessment on teachers' knowledge and understanding in no-fee public schools located in a low socioeconomic context. Aim: This article investigates the impact of the Assessment for Learning Capacity Development Programme (AfL CDP) on teachers' formative assessment knowledge and understanding pertaining to the five formative assessment strategies: learning intentions and success criteria, questioning, feedback, peer and self-assessment. Setting: This study was conducted as part of the Assessment for Learning (AfL) in Africa project in one Gauteng district involving 20 Grade 3 teachers from six no-fee public schools. Methods: Teachers in this study participated in the AfL CDP, implemented using the reflect, mediate, acquire and adapt, plan, prepare, present, support (ReMAPS) intervention framework. Baseline and endline data were collected using the formative assessment reflection exercises (FARE) before and after the AfL CDP, while t-tests were used to determine differences in performance. Results: The results revealed significant improvements in teacher formative assessment knowledge and understanding across all five strategies. Conclusion: The ReMAPS intervention framework, applied in the AfL CDP, proved successful in supporting teachers to improve their formative assessment knowledge and understanding, even when implemented in challenging contexts, and provides a viable, practical model for implementing AfL pedagogical strategy by the Department of Education. Contribution: This study adds to the body of knowledge by providing research-based findings about how an AfL capacity development programme implemented in a challenging context in South Africa, benefited teachers' pedagogical knowledge and understanding.