We report a rare case of adenosquamous carcinoma of the pancreas associated with humoral hypercalcemia of malignancy (HHM) in which parathyroid hormone-related protein (PTH-rP) was identified as the causative factor of hypercalcemia. A 72-year-old Japanese man was admitted to our institution complaining of fever and abdominal pain. Abdominal computed tomography demonstrated a large tumor in the body of the pancreas, with multiple liver metastases. Both serum calcium and PTH-rP levels were elevated. No accumulation was observed on bone scan with technetium-99. The patient died of pneumonia 3 months after admission. Autopsy demonstrated that the neoplasm in the pancreas showed an abrupt histological transition from adenocarcinoma to squamous cell carcinoma. PTH-rP was identified in the primary pancreatic tumor cells by immunohistochemical examination and a reverse-transcription polymerase chain reaction (RTPCR) method. We concluded that PTH-rP was the causative factor of the HHM, based on the laboratory data, immunohistochemical examination, and messenger RNA (mRNA) expression. This is a very rare report of adenosquamous cell carcinoma of the pancreas associated with HHM.