In anatomy, The lower esophageal sphincter is the muscular area where the esophagus meets the stomach. It is also known as the gastroesophageal sphincter and the cardiac sphincter. The sphincter serves as a one way valve. During peristalsis, the LES allows the food bolus to pass into the stomach. It prevents chyme, a mixture of bolus, stomach acid, and digestive enzymes, from returning up the esophagus. The LES is aided in the task of keeping the flow of materials in one direction by the diaphragm.

Deficiencies in the strength or the efficiency of the LES lead to various medical problems involving acid damage on the esophagus.