In anatomy of the digestive system, the duodenum is a hollow jointed tube that connects the stomach to the jejunum. It is the first part of the small intestine, and is about 18 cm (7 in) long. It starts with the duodenal bulb and ends at the ligament of Treitz. Two very important ducts open into the duodenum, namely the bile duct and the pancreatic duct.

The duodenum is divided into four sections. The first three sections form a "C" shape - the first part of the duodenum comes off the pylorus and makes a sharp ~90 degree turn to become the second part of the duodenum. The second part of the duodenum is where the pancreatic duct (and sometimes, separately, the common bile duct) empty into the gastrointestinal tract. The duodenum makes another sharp ~90 degree turn into the third part of the duodenum before joining with the jejunum. The duodenum is a retroperitoneal structure. The pH in the duodenum is 6.0 to 6.5 (slightly acidic).

The name duodenum is from the Latin duodenum digitorum, twelve fingers' breadths or inches.