Abstract Introduction/Objective Inactivation of SMARCA4 (BRG1) protein subunit of the SWI/SNF complex has been reported in various malignancies affecting multiple organs, often associated with a poor prognosis. We report a rare case of gastric SMARCA4-deficient carcinoma in a young patient, showing squamous differentiation and an aggressive clinical course. Methods/Case Report A 28-year-old male presented with epigastric pain and 15-lb weight loss over 3 months. Evaluation revealed a gastric cardia mass, liver lesion, and gastro-hepatic lymph-node. Biopsy of gastric and liver lesions revealed a poorly differentiated carcinoma. Following a course of neoadjuvant chemotherapy, and immunotherapy, total gastrectomy and distal esophagectomy were performed. Grossly, there was a 13 cm mass involving his gastric cardia and body, extending into his distal esophagus. On microscopy, the transmural tumor was arranged as solid sheets, syncytial aggregates, and nests with a focal pseudoglandular pattern in a lymphoplasmacytic rich stroma. Scattered bizarre giant cells were noted, along with brisk mitosis and focal necrosis. Focally, the tumor showed abundant glassy eosinophilic cytoplasm reminiscent of squamoid cells. Two of forty-two lymph nodes were positive for metastatic carcinoma. IHC showed focal positivity for AE1/3, OSCAR, CAM5.2, along with areas staining for p40 and p63; negative for CK7, CK20, CDX2, SATB2, synaptophysin, INSM1, SOX-10, LCA ERG, and SALL4. Nuclear expression for mismatch repair proteins was retained. HER2 was negative (score 0), and PDL-1 CPS score was 40. EBER in-situ hybridization was negative, with retention of INI-1. There was complete loss of SMARCA4 by IHC and next generation sequencing confirmed frameshift alteration of SMARCA4. Final pathologic diagnosis was SMARCA4 deficient poorly differentiated carcinoma with squamous differentiation (ypT3pN1). The patient received adjuvant therapy with Paclitaxel and Ramucirumab. However, his disease progressed, and he succumbed to his illness five months after surgery. Results (if a Case Study enter NA) N/A Conclusion Our case illustrates that SMARCA4 deficient gastric carcinomas can have heterogeneous morphology, with an aggressive clinical course in young patients. This highlights the importance of early diagnosis and consideration of alternative therapeutic approaches, as these tumors typically do not respond to standard treatments.