Papain, also known as papaya proteinase I, is a cysteine protease enzyme present in papaya (Carica papaya) and mountain papaya (Vasconcellea cundinamarcensis). It is the namesake member of the papain-like protease family.

Papain breaks down proteins, carbohydrates, and fats. People use papaya for cancer, diabetes, dengue fever, HPV, and many other conditions.