在古典逻辑中，无矛盾律（英语：Law of noncontradiction，缩写为LNC），也被称为矛盾律（law of contradiction），把断言命题 Q 和它的否定命题非-Q 二者同时在"同一方面"为真的任何命题 P 断定为假。用亚里士多德的话说，“你不能同时声称某事物在同一方面既是又不是”。

更简练的说，对于任何命题 P，P 和非-P 不能同时在场。在符号上，这可表达为:为真。

二值和有关规律检视了无矛盾律和类似定律的关系，比如二值原理，不应与之混淆。

This article uses forms of logical notation. For a concise description of the symbols used in this notation, see List of logic symbols.

In classical logic, the law of non-contradiction (LNC) (or the law of contradiction (PM) or the principle of non-contradiction (PNC), or the principle of contradiction) is the second of the three classic laws of thought. It states that contradictory statements cannot both be true in the same sense at the same time, e.g. the two propositions "A is B" and "A is not B" are mutually exclusive.