Which Supreme Court case made the exclusionary rule applicable to individual states?

The exclusionary rule, which prohibits the use of illegally obtained evidence in a court of law, was made applicable to individual states through the Supreme Court case of Mapp v. Ohio. In this landmark decision, the Court held that the exclusionary rule is necessary to enforce the Fourth Amendment's protection against unreasonable searches and seizures within the states. The Court determined that evidence obtained in violation of the Constitution cannot be used in state courts, thereby extending the protections of the Fourth Amendment and ensuring that individuals have recourse against unlawful actions by state law enforcement.

This case was pivotal in establishing that the rights guaranteed by the Bill of Rights are not just federal protections but are also applicable to state actions through the doctrine of incorporation, which applies the provisions of the Bill of Rights to state governments via the Fourteenth Amendment. The decision in Mapp v. Ohio thus set a precedent that significantly influenced future cases concerning search and seizure rights.