Background: SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) is a rare tumor that is independent of lung cancer and is classified as undifferentiated carcinoma of epithelial tumors according to the 2021 WHO classification of thoracic tumors. Loss of BRG1 expression due to SMARCA2 and SMARCA4 gene defects is a possible pathogenic mechanism. The pathological features are mainly nest-like rhabdoid cells with necrosis. Currently, there is a lack of well-recognized and convincing regimens for the treatment of SMARCA4-UT. Because the lesions are mainly located in the mediastinum and lung, patients can present with thoracic-related symptoms. We report the case of a middle-aged male patient with SMARCA4-UT who was admitted to the hospital with polyarthralgia and fever and lacked typical thoracic-related symptoms. This may be the first case report of SMARCA4-UT with fever and polyarthralgia as the initial symptoms, and its diagnostic idea and case characteristics are worthy of consideration and reference. Case Description: A 57-year-old male fisherman with a long history of smoking and emphysema was initially admitted to the Department of Rheumatology and Immunology because of fever and polyarthralgia as the initial symptoms. The imaging examination suggested a mediastinal mass. With the gradual advancement of the examination, the riddle of fever and polyarthralgia was gradually revealed. After symptom control with steroids, he underwent thoracoscopic surgery for mediastinal lymph node biopsy, and SMARCA4-UT was finally diagnosed. He was evaluated and transferred to the oncology department for one cycle of postoperative adjuvant chemotherapy, and he was discharged with good symptom control after discharge.We lost follow-ups of patients after they were discharged. Conclusions: Although fever and multiple arthralgias are not uncommon in neoplastic lesions, as atypical initial symptoms of rare tumors, clinicians should be aware of this when diagnosing tumors and not limited to SMARCA4-UT. In addition, the pathological and imaging features of SMARCA4-UT should be fully understood.