Informed consent is a principle in medical ethics and medical law and media studies, that a patient must have sufficient information and understanding before making decisions about their medical care. Pertinent information may include risks and benefits of treatments, alternative treatments, the patient's role in treatment, and their right to refuse treatment. In most systems, healthcare providers have a legal and ethical responsibility to ensure that a patient's consent is informed. This principle applies more broadly than healthcare intervention, for example to conduct research and to disclosing a person's medical information.

Within the US, definitions of informed consent vary, and the standard required is generally determined by the state. Informed consent requires a clear appreciation and understanding of the facts, implications, and consequences of an action. To give informed consent, the individual concerned must have adequate reasoning faculties and possess all relevant facts. Impairments to reasoning and judgment that may preclude informed consent include intellectual or emotional immaturity, high levels of stress such as post-traumatic stress disorder or a severe intellectual disability, severe mental disorder, intoxication, severe sleep deprivation, dementia, or coma.

Obtaining informed consent is not always required. If an individual is considered unable to give informed consent, another person is generally authorized to give consent on their behalf—for example, the parents or legal guardians of a child (though in this circumstance the child may be required to provide informed assent) and conservators for the mentally disordered. Alternatively, the doctrine of implied consent permits treatment in limited cases, for example when an unconscious person will die without immediate intervention. Cases in which an individual is provided insufficient information to form a reasoned decision raise serious ethical issues. When these issues occur, or are anticipated to occur, in a clinical trial, they are subject to review by an ethics committee or institutional review board.

Informed consent is codified in both national and international law. 'Free consent' is a cognate term in the International Covenant on Civil and Political Rights, adopted in 1966 by the United Nations, and intended to be in force by 23 March 1976. Article 7 of the covenant prohibits experiments conducted without the "free consent to medical or scientific experimentation" of the subject. As of September 2019, the covenant has 173 parties and six more signatories without ratification.