ReactOS is a free and open-source operating system for i586/amd64 personal computers that is intended to be binary-compatible with computer programs and device drivers developed for Windows Server 2003 and later versions of Microsoft Windows. ReactOS has been noted as a potential open-source drop-in replacement for Windows and has been of interest for its information on undocumented Windows APIs.

ReactOS has been in development since 1996. As of April 2025, it is still considered to be feature-incomplete alpha software. Therefore, it is recommended by the developers to be used only for evaluation and testing purposes. However, many Windows applications are working, such as Adobe Reader 9.3, GIMP 2.6, and LibreOffice 5.4.

ReactOS is primarily written in C, with some elements written in C++, such as the ReactOS File Explorer. The project partially implements Windows API functionality and has been ported to the AMD64 processor architecture. ReactOS is part of the FOSS ecosystem so it re-uses and collaborates with many other FOSS projects, most notably the Wine project that presents a Windows compatibility layer for Unix-like operating systems.