In anatomy of the digestive system, the jejunum is the central of the three divisions of the small intestine and lies between the duodenum and the ileum. In adult humans, it is usually between 2-8m long. The pH in the jejunum is usually between 7 and 8 (neutral or slightly alkaline).

The inner surface of the jejunum, its mucous membrane, is covered in projections called villi, which increase the surface area of tissue available to absorb nutrients from the gut contents. It differs from the duodenum due to lack of Brunner's glands. It is also different from the ileum due to less goblet cells and generally lacks Peyer's patches.

Jejunum is derived from the adjective jejune, which means "fasting" or "hungry" in Early Modern English.