Pseudocyst formation is a common complication of acute and chronic pancreatitis. Most common site of pseudocyst is lesser sac; mediastinal extension of pseudocyst is rare. Other possibilities of posterior mediastinal cyst must be considered. This patient presented with computed tomography abdomen with thorax showing a large thoraco-abdominal pseudocyst with right sided pleural effusion. It was confirmed to be pancreatic pseudocyst by analyzing fluid for amylase and lipase during surgery. In our patient, the pseudocyst was accessible transabdominaly. Cystogastrostomy was not possible as it was causing twisting of cardio-esophageal junction; we did retrocolic and retrogastric Roux-en-Y cystojejunostomy. Only two such cases were reported in literature. Pseudocyst formation is a common complication of acute and chronic pancreatitis. Most common site of pseudocyst is lesser sac; mediastinal extension of pseudocyst is rare. Other possibilities of posterior mediastinal cyst must be considered. This patient presented with computed tomography abdomen with thorax showing a large It was confirmed to be pancreatic pseudocyst by analyzing fluid for amylase and lipase during surgery. In our patient, the pseudocyst was accessible transabdominaly. Cystogastrostomy was not possible as it was caused caused twophage-cardio-esophageal junction; we did retrocolic and retrogastric Roux-en-Y cystojejunostomy. Only two such cases were reported in literature.