In criminal law, an overt act is the one that can be clearly proved by evidence and from which criminal intent can be inferred, as opposed to a mere intention in the mind to commit a crime. Therefore, it is an act that, while innocent per se, can potentially be used as evidence against someone during a trial to show participation in a crime. For instance, the purchase of a ski mask, which can conceal identity, may be an overt act in the planning of a bank robbery.
The term is more particularly employed in cases of treason, which must be demonstrated by some overt or open act.
This rule derives from the Treason Act 1695, passed by the Parliament of England and adopted by the United States in Article III, Section 3 of the United States Constitution, which provides that "No Person shall be convicted of Treason unless on the Testimony of two Witnesses to the same overt Act, or on Confession in open Court." In Cramer v. United States, the Supreme Court ruled that "every act, movement, deed, and word of the defendant charged to constitute treason must be supported by the testimony of two witnesses." In Haupt v. United States, however, the Supreme Court found that two witnesses are not required either to prove intent or to prove that an overt act is treasonable. The two witnesses, according to the decision, are required to prove only that the overt act occurred.
In criminal conspiracy cases, an overt act must frequently be proven to convict the defendants of the crime.
Many conspiracy theorists claim that documents saying that the United States desired that Japan to commit the first "overt act of war" gives credit to Pearl Harbor advance-knowledge conspiracy theories.