二补数（2's complement）是一种用二进位表示有号数的方法，也是一种将数字的正负号**的方式，常在计算机科学中使用。在中国称作补码，**和香港称为二补数。

一个数字的二补数就是将该数字作比特反相运算（即一补数或反码），再将结果加1。在二补数系统中，一个负数就是用其对应正数的二补数来表示。

二补数系统的最大优点是可以在加法或减法处理中，不需因为数字的正负而使用不同的计算方式。只要一种加法电路就可以处理各种有号数加法，而且减法可以用一个数加上另一个数的二补数来表示，因此只要有加法电路及二补数电路即可完成各种有号数加法及减法，在电路设计上相当方便。

Two's complement is a mathematical operation on binary numbers, as well as a binary signed number representation based on this operation. Its wide use in computing makes it the most important example of a radix complement.

The two's complement of an N-bit number is defined as the complement with respect to 2; in other words, it is the result of subtracting the number from 2. This is also equivalent to taking the ones' complement and then adding one, since the sum of a number and its ones' complement is all 1 bits. The two's complement of a number behaves like the negative of the original number in most arithmetic, and positive and negative numbers can coexist in a natural way.