在范畴论中，米田引理断言一个对象的性质由它所表示的函子或决定。此引理得名于日本数学家暨计算机科学家米田信夫。

设为一范畴，定义两个函子范畴如下：

并定义两个函子：

其中而。

米田引理的抽象陈述如下：

米田引理。有自然的同构

这两个同构对所有变元都满足函子性。

对任一对象，在上述同构中分别取，便得到米田引理最常见的形式：

推论。函子与是完全忠实的。

In mathematics, specifically in category theory, the Yoneda lemma is an abstract result on functors of the type morphisms into a fixed object. It is a vast generalisation of Cayley's theorem from group theory (viewing a group as a particular kind of category with just one object). It allows the embedding of any category into a category of functors (contravariant set-valued functors) defined on that category. It also clarifies how the embedded category, of representable functors and their natural transformations, relates to the other objects in the larger functor category. It is an important tool that underlies several modern developments in algebraic geometry and representation theory. It is named after Nobuo Yoneda.