Fruit of the poisonous tree is a legal metaphor used to describe evidence that is obtained illegally. The logic of the terminology is that if the source (the "tree") of the evidence or evidence itself is tainted, then anything gained (the "fruit") from it is tainted as well.

The doctrine underlying the name was first described in Silverthorne Lumber Co. v. United States, 251 U.S. 385 (1920). The term's first use was by Justice Felix Frankfurter in Nardone v. United States (1939).

Such evidence is not generally admissible in court. For example, suppose a police officer obtained a key to a train station locker in the process of conducting a search of a home that was unconstitutional on the grounds that it violated the Fourth Amendment. Any evidence of a crime that came from that locker would most likely be excluded under the "fruit of the poisonous tree" legal doctrine. See also United States ex rel. Chennault v. Smith (1973). The testimony of a witness who is discovered through illegal means would not necessarily be excluded, however, due to the "attenuation doctrine", which allows certain evidence or testimony to be admitted in court if the link between the illegal police conduct and the resulting evidence or testimony is sufficiently attenuated. For example, a witness who freely and voluntarily testifies is enough of an independent intervening factor to sufficiently "attenuate" the connection between the government's illegal discovery of the witness and the witness's voluntary testimony itself (United States v. Ceccolini, 435 U.S. 268 (1978)).

The "fruit of the poisonous tree" doctrine is an extension of the exclusionary rule, which, subject to some exceptions, prevents evidence obtained in violation of the Fourth Amendment from being admitted in a criminal trial.^{[citation needed]} Like the exclusionary rule, the fruit of the poisonous tree doctrine is intended to deter police from using illegal means to obtain evidence.^{[citation needed]}

The doctrine is subject to four main exceptions.^{[citation needed]} The tainted evidence is admissible if:

The American doctrine of the fruit of the poisonous tree has generally been rejected by the courts and legislators in Australia. Courts have tended to reject evidence where there is serious risk of unreliability, but where evidence is obtained unlawfully or improperly, the interest in deterring the police from unlawful, improper, or unfair treatment of the accused is balanced against the public interest in ensuring that those who commit crimes are brought to justice, and the seriousness of the crimes committed. Improperly obtained evidence is therefore several times more likely to be excluded from less serious offences like drug possession or disorderly conduct than from more serious ones like robbery and murder.

There is no comparable rule in English law. English courts have relied on the Court of Appeal case R v Leathem (1861) 8 Cox CC 498 to admit evidence irrespective of the legality of the source. This is the general stance.

It matters not how you get it; if you steal it even, it would be admissible in evidence.