毒树果实理论（英语：Fruit of the poisonous tree）在美国指的是调查过程中，透过非法手段的取得的证据，该术语的逻辑是如果证据的来源（树）受到污染，那么任何从它获得的证据（果实）也是被污染的，在诉讼审理的过程中将不能被采纳，即使该证据足以扭转裁判结果亦然。

例如，一位警察对住家进行违宪（美国宪法第四修正案）搜索，取得了一把车站储物柜的钥匙，之后发现的犯罪证据亦来自该储物柜，那么这个证据便很有可能因为毒树果实理论而被排除。但证人的发现本身并不算是证据，因为在法庭上的证词是经由独立的讯问而来，毒性已经衰减。

Fruit of the poisonous tree is a legal metaphor in the United States 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 fruit of the poisonous tree doctrine 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).