非形式谬误（英语：informal fallacies）意指一个推理犯了论证结构（形式）以外的错误。

非形式谬误有几种常见的表现方式：

非形式谬误由于以上几种情形，使得论证缺乏合理性或说服力。我们需要检验论据的具体内容，才能确认非形式谬误的存在，这往往需要关于现实世界的背景知识与常识。

归纳性谬误皆属非形式谬误；演绎性谬误则是形式谬误。

An informal fallacy occurs when the contents of an argument's stated premises fail to adequately support its proposed conclusion. In contrast to a formal fallacy of deduction, the error is not a flaw in the form of the argument. Though the form of the argument may be relevant, it is also the content that is implicated in the erroneous reasoning. So while formal fallacies always guarantee that the resulting argument is invalid, an argument containing an informal fallacy might employ a valid logical form while nevertheless remaining rationally unpersuasive.