向下兼容（downward compatibility），又称向后兼容（backward compatibility）、回溯兼容，在计算机中指在一个进程、库或硬件更新到较新版本后，用旧版本进程创建的文档或系统仍能被正常操作或使用（包括写入）、在旧版本库的基础上开发的进程仍能正常编译运行，或较旧版的硬件仍可在新版使用的情况。2015秋季，家用游戏机Xbox One开放使用向下兼容游玩Xbox 360，支持的游戏日渐增加。

In telecommunications and computing, a product or technology is backward compatible (BC) or downward compatible if it can function properly given input generated by, or meant for, an older product or technology, such as a legacy system. If products designed for the new standard can receive, read, view or play older standards or formats, then the product is said to be backward-compatible; examples of such a standard include data formats and communication protocols. Modifications to a system that do not allow backward compatibility are sometimes called "breaking changes."