A woman in her 20s visited our hospital with a chief complaint of abdominal distension and back pain. She was diagnosed with a cystic tumor (diameter 16 cm) in the tail of the pancreas and underwent a combined distal pancreatectomy, splenectomy, and left adrenalectomy. Histopathologically, the tumor presented as a mucinous cystic neoplasm with an undifferentiated carcinoma component of the pancreas. In addition, the cells demonstrated a partial rhabdoid-like morphology. These findings were considered relatively typical for a mucinous cystic neoplasm in the tail of the pancreas in a young woman. However, NRAS mutation, which is rare in pancreatic tumors, was detected.