BackgroundAnaplastic thyroid cancer (ATC) is a rare but highly fatal form of thyroid cancer. This highly malignant tumor progresses rapidly and is prone to relapse and metastasis, with a poor prognosis. Novel treatments have improved survival in recent years, but the outcome of treatment is not satisfactory.Case presentationWe report a case of multiple postoperative recurrences of papillary thyroid carcinoma that later transformed into undifferentiated carcinoma. The patient’s neck mass was huge and the operation was unsuitable. Then, she achieved remarkable tumor shrinkage by tislelizumab immunotherapy combined with radiotherapy.ConclusionThis case indicates that radiotherapy combined with immunotherapy is a promising treatment for ATC. Such a combined approach warrants further study.