在数论中，欧几里得引理是根据欧几里得的《几何原本》第七卷的命题30推出的一个定理。这个引理说明：

可以这样表达这个引理：

命题30是这样说的：

如果一个素数整除两个正整数的乘积，那幺这个素数可以至少整除这两个正整数中的一个。

In number theory, Euclid's lemma is a lemma that captures a fundamental property of prime numbers, namely: If a prime divides the product of two numbers, it must divide at least one of those numbers. It is also called Euclid's first theorem although that name more properly belongs to the side-angle-side condition for showing that triangles are congruent. For example, 133 × 143 = 19019, and since 19019 is divisible by 19, the lemma implies that one or both of 133 or 143 must be as well. In fact, 133 = 19 × 7.