Undifferentiated pleomorphic sarcoma (UPS) is a rare and aggressive soft tissue tumor with a high degree of malignancy and rapid progression, usually occurring in the extremities, retroperitoneum, and abdomen, whereas it rarely arises in the mediastinum, and is treated mainly by surgical resection. The prognosis of patients with advanced sarcoma is poor, and doxorubicin monotherapy is the standard first-line chemotherapy for most advanced soft tissue sarcomas (STS), but the prognosis is generally unsatisfactory. Immune checkpoint inhibitors (ICIs) have been established as therapies for many solid cancers in recent years; however, evidence on the efficacy of ICIs in undifferentiated sarcoma is scarce, mostly consisting of small studies, and no ICIs are currently approved for use in sarcomas. We report a case of a middle-aged man with primary mediastinal UPS with high PD-L1 expression (TPS was approximately 80%) and TLS positive. The patient was treated with sequential tislelizumab monotherapy maintenance after 6 cycles of tislelizumab combined with epirubicin, efficacy evaluation was partial remission (PR), progression-free survival (PFS) was 8.5 months, and grade 1 fatigue was identified as an adverse event.