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Pancreatic adenosquamous carcinoma (PASC) is a rare and highly aggressive subtype of pancreatic cancer with a dismal prognosis, and its preoperative diagnosis remains challenging. We report the case of an 82-year-old man who presented with epigastric pain and appetite loss. Imaging studies revealed a large, partially necrotic mass in the tail of the pancreas that was contiguous with the posterior gastric wall, accompanied by multiple metastases to the liver, lungs, regional lymph nodes, and peritoneum. Upper gastrointestinal endoscopy identified a large, excavated ulcer within a submucosal tumor-like elevation, and biopsies from the ulcer margin confirmed the presence of squamous cell carcinoma. The pathological findings showed PASC. The patient was diagnosed with Stage Ⅳ PASC, and palliative care was selected in view of advanced dementia and disease extent. This case shows that PASC can present with direct gastric penetration, and it highlights the value of direct endoscopic biopsy of the invaded gastric site to secure a definitive preoperative histopathologic diagnosis. PASC should be considered in the differential diagnosis of pancreatic masses that exhibit gastric invasion and marked internal necrosis on imaging. J. Med. Invest. 73 : 286-290, February, 2026.