In the United States, the exclusionary rule is a legal rule, based on constitutional law, that prevents evidence collected or analyzed in violation of the defendant's constitutional rights from being used in a court of law. This may be considered an example of a prophylactic rule formulated by the judiciary in order to protect a constitutional right. The exclusionary rule may also, in some circumstances at least, be considered to follow directly from the constitutional language, such as the Fifth Amendment's command that no person "shall be compelled in any criminal case to be a witness against himself" and that no person "shall be deprived of life, liberty or property without due process of law."

The exclusionary rule is grounded in the Fourth Amendment in the Bill of Rights, and it is intended to protect citizens from illegal searches and seizures. The exclusionary rule is also designed to provide a remedy and disincentive for criminal prosecution from prosecutors and police who illegally gathered evidence in violation of the Fifth Amendment and its protection against self-incrimination. The exclusionary rule also protects against violations of the Sixth Amendment, which guarantees the right to counsel.

Most states also have their own exclusionary remedies for illegally obtained evidence under their state constitutions and/or statutes, some of which predate the federal constitutional guarantees against unlawful searches and seizures and compelled self-incrimination.

This rule is occasionally referred to as a legal technicality because it allows defendants a defense that does not address whether the crime was actually committed. In this respect, it is similar to the explicit rule in the Fifth Amendment protecting people from double jeopardy. In strict cases, when an illegal action is used by police/prosecution to gain any incriminating result, all evidence whose recovery stemmed from the illegal action—this evidence is known as "fruit of the poisonous tree"—can be thrown out from a jury (or be grounds for a mistrial if too much information has been irrevocably revealed).

The exclusionary rule applies to all persons within the United States regardless of whether they are citizens, immigrants (legal or illegal), or visitors.