The mucous membranes (or mucosae; singular mucosa) are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs. They are at several places contiguous with skin: at the nostrils, the mouth, the lips, the eyelids, the ears, the genital area, and the anus. The sticky, thick fluid secreted by the mucous membranes and glands is termed mucus. The term mucous membrane refers to where they are found in the body and not every mucous membrane secretes mucus.

The glans clitoridis, glans penis (head of the penis), along with the inside of the foreskin and the clitoral hood, are mucous membranes. The urethra is also a mucous membrane. The secreted mucus traps the pathogens in the body, preventing any further activities of diseases.