in U.S. law, the principle that evidence seized by police in violation of the Fourth Amendment to the U.S. Constitution may not be used against a criminal defendant at trial.

The Fourth Amendment guarantees freedom from unreasonable searches and seizures—that is, those made without a warrant signed by a judge. The U.S. Supreme Court held in Wolf v. Colorado (1949) that “security of one's privacy against arbitrary intrusion by the police—which is at the core of the Fourth Amendment—is basic to a free society.” However, that decision did not extend to state courts. During the next decade, approximately half of the states adopted the rule. Later the Supreme Court held in Mapp v. Ohio (1961) that the rule had to be applied universally to all criminal proceedings.

The broad provisions of the exclusionary rule came under legal attack, and in U.S. v. Leon (1984) the Supreme Court held that evidence obtained “in good faith” with a search warrant later ruled invalid was admissible. A central argument was the unacceptable social cost of excluding such evidence, a reason subsequently given for creating further exceptions to the rule.