在数学、语义学和语言哲学中，复合性原理是指，一个复杂表达式的意义是由其各组成部分的意义以及用以结合它们的规则来决定的。

复合性原理认为，在有意义的句子中，如果实词部分被从句子中脱离出去，所剩下的东西就是复合的规则。例如句子“苏格拉底是人”。一旦有意义的词法项“苏格拉底”和“人”被取走，剩下了伪句子“S 是 M”。其作用为描述 S 和 M 之关系。

这常意味着每个语法运算均应与一个语义运算相对应，后者作用于由该语法运算结合的各组成部分的意义之上。作为构造语义理论的原理，在唐纳德·戴维森的有影响力的语言哲学著作中，它一般意味着所有的语法构造应当关联着T-模式的从句，它带有在指定整个表达式如何通过语法规则构造自组件的语义中的一个算子。在某些一般数学理论（特别是符合蒙塔古语法传统的）中，这个原理意味着要理解一种语言，则必须给出其语法表现及其语义对象的代数结构之间的同态。

In mathematics, semantics, and philosophy of language, the principle of compositionality is the principle that the meaning of a complex expression is determined by the meanings of its constituent expressions and the rules used to combine them. This principle is also called Frege's principle, because Gottlob Frege is widely credited for the first modern formulation of it. However, the idea appears already among Indian philosophers of grammar such as Yāska, and also in Plato's work such as in Theaetetus. Besides, the principle was never explicitly stated by Frege, and it was arguably already assumed by Boole decades before Frege’s work.