Munchausen syndrome by proxy (MSP), a type of factitious disorder, is a mental illness in which a person acts as if an individual he or she is caring for has a physical or mental illness when the person is not really sick. The adult perpetrator has MSP and directly produces or lies about illness in another person under his or her care, usually a child under 6 years of age. It is considered a form of abuse by the American Professional Society on the Abuse of Children. However, cases have been reported of adult victims. (The term "by proxy" means "through a substitute.")

People with MSP have an inner need for the other person (often his or her child) to be seen as ill or injured. It is not done to achieve a concrete benefit, such as financial gain. People with MSP are even willing to have the child or patient undergo painful or risky tests and operations in order to get the sympathy and special attention given to people who are truly ill and their families. Factitious disorders are considered mental illnesses because they are associated with severe emotional difficulties.