Allman v. United States was a United States Supreme Court case that dealt with the issue of whether a defendant could be convicted of a crime if the evidence presented at trial was obtained through an illegal search and seizure. The case involved a man named Allman who was charged with receiving stolen goods. The evidence used to convict him was obtained through a search of his home without a warrant. The Supreme Court held that the evidence obtained through the illegal search and seizure was inadmissible and that Allman could not be convicted on the basis of it. The Court reasoned that the Fourth Amendment of the United States Constitution protects citizens from unreasonable searches and seizures and that the evidence obtained through the illegal search and seizure was in violation of this amendment. The Court also held that the exclusionary rule applied in this case, meaning that any evidence obtained through an illegal search and seizure was inadmissible in court. The decision in Allman v. United States established the precedent that evidence obtained through an illegal search and seizure is inadmissible in court and cannot be used to convict a defendant. This decision has been cited in numerous cases since then and has become an important part of Fourth Amendment jurisprudence.

In Allman v. United States, the Supreme Court was asked to decide whether a federal statute that allowed for the forfeiture of property used in connection with certain crimes applied to an individual who had been convicted of such a crime but whose property had not yet been seized by authorities. The majority opinion held that it did, finding that Congress intended for this law to apply even when the property has not yet been taken into custody. Justice Field dissented from this decision, arguing that there was no evidence in either the text or legislative history of the statute indicating any intention on behalf of Congress to allow forfeitures prior to seizure and possession by government officials. He further argued that allowing pre-seizure forfeitures would be contrary both to common sense and established legal principles regarding criminal procedure and due process rights under the Fifth Amendment.