非单调逻辑（英语：Non-monotonic logic）是（在前提的集合和单一的句子之间的）推论关系不是单调递增的形式逻辑。

与单调推理（经典逻辑）相对，非单调推理是指知识库加入新知识后，原有的推论会被推翻的逻辑。也就是说，知识库的推论不随着知识增长而增长，即非单调递增。这时，必须使用某种正确的维持机制，确保推理继续进行。因此，非单调推理多是在知识不完全的情况下发生的。

多数形式逻辑都有单调性的推论关系，就是说，如果一个句子可以从前提的集合中推理出来，则它也可以从把这个前提集合作为子集包含的任何前提集合中推理出来，这意味着向理论增加一个公式永不引起它的推论集合的减小。在直觉上，单调性指示出学习一些新知识不能减小已知知识的集合。单调逻辑不能处理各种推理任务比如缺省推理（事实可以是已知的，只是因为缺乏反面的证据）、溯因推理（事实只按最合适的解释演绎出来）、关于知识的推理（在事实变成已知的时候，对一个事实的无知必须被撤消），和信念修正（新知识可以和旧信念矛盾。）

A non-monotonic logic is a formal logic whose consequence relation is not monotonic. In other words, non-monotonic logics are devised to capture and represent defeasible inferences (c.f. defeasible reasoning), i.e., a kind of inference in which reasoners draw tentative conclusions, enabling reasoners to retract their conclusion(s) based on further evidence. Most studied formal logics have a monotonic consequence relation, meaning that adding a formula to a theory never produces a reduction of its set of consequences. Intuitively, monotonicity indicates that learning a new piece of knowledge cannot reduce the set of what is known. A monotonic logic cannot handle various reasoning tasks such as reasoning by default (consequences may be derived only because of lack of evidence of the contrary), abductive reasoning (consequences are only deduced as most likely explanations), some important approaches to reasoning about knowledge (the ignorance of a consequence must be retracted when the consequence becomes known), and similarly, belief revision (new knowledge may contradict old beliefs).