Pancreatitis is an inflammatory pancreatic disease; common etiologies include infection, anatomic abnormalities, biliary, inborn errors of metabolism, trauma, and rarely malignancy. Primary mediastinal large B-cell lymphoma commonly presents in younger women with principally mediastinal involvement. We report the first documented case of a pediatric patient presenting with acute pancreatitis secondary to metastatic primary mediastinal large B-cell lymphoma. Since diagnosis, the patient underwent a combination of chemotherapy and immunotherapy treatments, and the tumor burden had decreased significantly. Malignancy is a rare documented presentation of acute pancreatitis in the pediatric population, and it should be included in a broad differential diagnosis.