A 62-year-old man was admitted with dyspnea. Computed tomography (CT) revealed left massive pleural effusion and a cystic lesion in the posterior mediastinal compartment extending to the pancreatic head via the esophageal hiatus. The pleural effusion had a high amylase content. Based on these findings, we diagnosed mediastinal pancreatic pseudocyst accompanied by pancreatic pleural effusion. We treated him with CT-guided puncture and endoscopic pancreatic drainage. Endoscopic pancreatic treatment is possible for pancreatic pseudocysts.