The cecum is part of the digestive system of mammals. It is the first part of the colon. It is in the form of a pouch or sac following the ileum of the small intestine. It is separated from the ileum by the ileo-caecal valve. The ileo-caecal valve regulates the entry of the food bolus into the caecum, which then passes the digesting food to the ascending colon. The appendix, whose inflammation can lead to appendicitis, is located in the caecum.

The main role of the cecum is to absorb the water and electrolytes still present after digestion and absorption in the small intestine. The ileo-caecal valve and sphincter usually prevent the return of material to the ileum. This one-way barrier is essential to prevent contamination of the small intestine by bacteria present in the colon.