This study explored how secondary school EFL teachers engaged in professional learning opportunities when they participated in the development of ELT materials. While increasing research has examined language teaching materials and teacher learning, the two research fields have remained largely disconnected, hindering the potential of materials writing for professional learning. With eight teachers as focal participants, the data included in-depth interviews, written reflections, and official documents. Findings showed that the EFL teachers took advantage of materials writing for their professional learning in the areas of policy, theory, and practice. Their engagement with the national curriculum standards, language, and language learning theories, together with local classroom realities in relation to teaching materials, reciprocally interacted with their professional learning. Based on the findings, we recommend that professional development programs include materials writing by localizing the content, clarifying its objectives, teaming teachers with researchers, and creating feedback loops.