换质换位律（contraposition, transposition），又称异质位换律、换质位法，是古典逻辑的一种结构变换推理，一般用于改变条件命题的结构。

在直言命题中，换质换位律只能用于全称肯定型（A型）和特称否定型（O型），而不能用在全称否定型（E型）和特称肯定型（I型）；套用在后者是非法的换质换位（illicit contraposition），是一种形式谬误。

In logic, contraposition is a law that says that a conditional statement is logically equivalent to its contrapositive. The contrapositive of the statement has its antecedent and consequent inverted and flipped: the contrapositive of is thus . For instance, the proposition "All bats are mammals" can be restated as the conditional "If something is a bat, then it is a mammal". Now, the law says that statement is identical to the contrapositive "If something is not a mammal, then it is not a bat."