Thoracic SMARCA4-deficient undifferentiated tumors (SMARCA4-UT) are aggressive neoplasms that most commonly occur in the mediastinum of male smokers. These tumors are characterized by an inactivating mutation of SMARCA4 resulting in loss of expression of brahma-related gene 1 (BRG1). These tumors can have a variable immunophenotype but in general have no or only focal keratin expression and characteristically lack expression of BRG1. Most patients have metastatic disease at time of presentation. Usually SMARCA4-UT progress or recur and the median survival of these patients is only approximately half a year. Preclinical and clinical trials using enhancer of zeste homolog (EZH2) inhibitors are underway to potentially treat this neoplasm. In addition, rare cases of successful treatment with anti-PD-1 inhibitors are described. Here, the case of a 66-year-old male smoker who presents with mediastinal and left suprahilar masses and widespread metastatic disease is reported. A biopsy reveals extensive necrosis and clusters and small sheets of neoplastic epithelioid cells with some exhibiting rhabdoid cytology. The tumor cells lack staining with various keratins and markers of lymphoid, melanocytic, myogenic, or vascular differentiation. Focal expression of CD30 is noted. BRG1 expression is lost in the tumor cells while INI-1 expression is preserved. This tumor is diagnosed as SMARCA4-UT.