BACKGROUND: Thymic epithelial tumours (TETs) are rare tumours, and most patients undergo curative resection. A minority require post-operative radiotherapy (PORT); however, current published guidelines provide differing recommendations. With few cases, a new staging system, increasing use of minimally invasive operative techniques, and challenging histopathology, postoperative radiotherapy planning is increasingly difficult with significant national variation. To bridge this gap, the multidisciplinary British Thoracic Oncology Group (BTOG) Thymic Malignancies Special Interest Group convened a working group to produce a user-friendly, practical guideline for TET postoperative radiotherapy. METHODS: Through synthesis of current published guidelines, expert multidisciplinary opinions (radiologists, surgeons, histopathologists, and clinical oncologists), and a blinded clinical target volume (CTV) contouring assessment, areas of agreement and best practice were identified. RESULTS: Subsequently, mandatory, and optional, recommendations were agreed for all steps of the postoperative radiotherapy planning process, including which areas to treat, radiotherapy planning techniques, and radiotherapy dose fractionation. CONCLUSION: Implementation of this national guideline will improve clinician knowledge related to postoperative radiotherapy for TETs and should reduce unwarranted variations in practice.