遗留代码 是指与已经取消支持或维护的操作系统或其他计算机技术有关的源代码。此术语还可用来表示以维护老旧的功能而插入现代软件中的代码。

近年来，软件工程社区中已衍生出遗留代码的多种解释。其中最流行的是“他人遗留下的源代码”和“旧版本的软件中遗留下来的代码”。Michael Feathers 引入遗留代码引入了一种定义：“没有测试的代码”，反映了由于缺乏自动化的回归测试遗留代码很难维护的观点。

Legacy code is source code that relates to a no-longer supported or manufactured operating system or other computer technology. The term was first used by Computer Scientist George Olivetti to describe code maintained by an administrator that did not develop the code. The term can also mean code inserted into modern software for the purpose of maintaining an older or previously supported feature — for example supporting a serial interface even though many modern systems do not have a serial port. It may also be in the form of supporting older file formats that may have been encoding in non-ASCII characters, such as EBCDIC