说谎者悖论，在哲学和逻辑学中，古典的说谎者悖论是指一个说谎者声称自己正在说谎：例如，声称：「我正在说谎」或者「我所说的皆为假」。如果他确实在说谎，那么他所说的就是真的，但如果他所说的就是真的，那么他就是在说谎。在「这个语句正在说谎」的悖论中，为了强化悖论，使悖论更经得起严格的逻辑分析，「说谎」的概念往往被「真假」的概念所取代，仅仅保留「说谎者」这一名称来指涉关于古典二值逻辑会推导出矛盾的悖论。

如果「这个语句为假」为真，那么这个语句为假，但是如果这个语句声称它为假，且它为假，那么它一定为真，如此一来悖论于焉成形。

In philosophy and logic, the classical liar paradox or liar's paradox is the statement of a liar who states that they are lying: for instance, declaring that "I am lying" or "everything I say is false". If they are indeed lying, they are telling the truth, which means they are lying. In "this sentence is a lie" the paradox is strengthened in order to make it amenable to more rigorous logical analysis. It is still generally called the "liar paradox" although abstraction is made precisely from the liar theirself. Trying to assign to this statement, the strengthened liar, a classical binary truth value leads to a contradiction.