A legacy system is an old method, technology, computer system, or application program that continues to be used, typically because it still functions for the users' needs, even though newer technology or more efficient methods of performing a task are now available. A legacy system may include procedures or terminology which are no longer relevant in the current context, and may hinder or confuse understanding of the methods or technologies used.

The term "legacy" may have little to do with the size or age of the system — mainframes run 64-bit Linux and Java alongside 1960s vintage code.

Although the term is most commonly used to describe computers and software, it may also be used to describe human behaviors, methods, and tools. For example, timber framing using wattle and daub is a legacy building construction method.

Organizations can have compelling reasons for keeping a legacy system, such as:

NASA's Space Shuttle program still uses a large amount of 1970s-era technology. Replacement is cost-prohibitive because of the expensive requirement for flight certification; the legacy hardware currently being used has completed the expensive integration and certification requirement for flight, but any new equipment would have to go through that entire process – requiring extensive tests of the new components in their new configurations – before a single unit could be used in the Space Shuttle program. This would make any new system that started the certification process a de facto legacy system by the time of completion.