Mucosa is a specialized lining tissue. It consists mainly of columnar epithelial cells, and mainly lines the lumens of the body, such as the respiratory and digestive tracts. Mucosa has the ability to produce mucous secretions. This property helps to keep delicate tissues moist and well-protected. There are several specialized types of mucosal tissue in, for example, the stomach and small intestine.

The most easily examined mucosa is the lining of the cheeks (buccal mucosa) and mouth (oral mucosa), the inner surfaces of the eyelids, the inside of the nostrils and nasal tracts, and the rectum. The vermillion border marks the transition from skin to mucosa at the mouth, as does the anal verge at the other end.