Objective To analyze the clinical and CT characteristics of primary thoracic SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) and improve recognition of this disease. Methods A retrospective analysis was conducted on 21 cases of pathologically confirmed primary thoracic SMARCA4-UT treated at our institution from July 2018 to August 2024. Clinical and CT features were evaluated, and we conducted a literature review. Results Clinical features of SMARCA4-UT: There were 20 (95.2%) male patients and 1 (4.8%) female patient, aged from 43 to 78 years, with a mean age of 59.9 years. 16 cases of the 21 patients (76.2%) were smokers, with smoking duration ranging from 20 to 50 years (mean 36 years). The main clinical symptoms were cough and expectoration (11 cases, 52.4%), chest pain and chest tightness (9 cases, 42.8%), etc. CT features of SMARCA4-UT: In this study, 7 cases (33.3%) were located outside the lung parenchyma, such as mediastinum, hilar or pleura, and 14 cases (66.7%) were located in the lung parenchyma. 20 cases (95.2%) showed nodule or mass like growth. Lobulation was seen in 20 cases (95.2%). 13 cases (61.9%) had unclear boundaries. 13 cases (61.9%) invaded adjacent structures. 18 cases (85.7%) had lymph node metastasis or another organ metastasis. CT plain scan showed low density in 21 cases. 20 cases (95.2%) showed cystic necrosis and uneven density. No calcification was found in 21 cases. Contrast-enhanced scan was performed in 18 cases, and 14 cases (77.8%) showed mild enhancement. 5 cases underwent 18F-FDG-PET/CT examination, all of which showed high uptake, with an average SUV max of 10.1. Conclusion SMARCA4-UT predominantly affects elderly males with heavy smoking history. The clinical manifestations are nonspecific, and the lesions are mostly located in the lung parenchyma. On CT scan, the density is often heterogeneous, necrosis is common, and the enhancement is usually mild. On PET-CT, FDG uptake is high in all cases. Most cases are accompanied by lymph node or other organ metastasis at the first diagnosis. Combined clinical and radiological analysis is essential for accurate diagnosis