Mens rea; Law Latin for "guilty mind") is the mental element of a person's intention to commit a crime; or knowledge that one's action or lack of action would cause a crime to be committed. It is a necessary element of many crimes.

The standard common law test of criminal liability is expressed in the Latin phrase actus reus non facit reum nisi mens sit rea, i.e. "the act is not culpable unless the mind is guilty". In jurisdictions with due process, there must be both actus reus ("guilty act") and mens rea for a defendant to be guilty of a crime (see concurrence). As a general rule, someone who acted without mental fault is not liable in criminal law. Exceptions are known as strict liability crimes. Moreover, when a person intends a harm, but because of bad aim or other cause, the intent is transferred from an intended victim to an unintended victim, the case is considered to be a matter of transferred intent.

In civil law, it is usually not necessary to prove a subjective mental element to establish liability for breach of contract or tort, for example. But if a tort is intentionally committed or a contract is intentionally breached, such intent may increase the scope of liability and the damages payable to the plaintiff.