United States v. Wiley was a United States Supreme Court case that was decided in 1870. The case involved a dispute between the United States and Wiley, a former employee of the United States Treasury Department. Wiley had been accused of embezzling funds from the Treasury Department and was subsequently indicted by a grand jury. At trial, Wiley argued that the indictment was invalid because it had been issued by a grand jury that had been improperly selected. The Supreme Court disagreed and held that the indictment was valid. The Court reasoned that the selection of the grand jury was not a matter of constitutional concern and that the indictment was valid as long as it was issued by a grand jury that was properly impaneled. The Court also held that the indictment was valid even though it was based on evidence that had been obtained through an illegal search. The Court reasoned that the exclusionary rule, which prohibits the use of illegally obtained evidence in criminal proceedings, did not apply in this case because the evidence had been obtained prior to the adoption of the rule. In the end, the Supreme Court affirmed the conviction of Wiley and held that the indictment was valid. The Court's decision established that the selection of a grand jury is not a matter of constitutional concern and that the exclusionary rule does not apply to evidence obtained prior to its adoption.

In United States v. Wiley, the Supreme Court was asked to decide whether a defendant could be convicted of violating an act of Congress when the act did not explicitly state that it was criminal in nature. The majority opinion held that such convictions were permissible as long as there was evidence that Congress intended for its acts to have criminal consequences. Justice Field dissented from this decision and argued that it violated basic principles of due process by allowing individuals to be punished without clear notice or warning about what conduct is prohibited. He further argued that if Congress wanted certain actions to be punishable, then they should make their intentions explicit in order for citizens to know which behaviors are subject to punishment under federal law.