MPlayer is a free and open source media player. The program is available for all major operating systems, including Linux and other Unix-like systems, Microsoft Windows and Mac OS X. Versions for OS/2, Syllable, AmigaOS, MorphOS and AROS Research Operating System are also available. The Windows version works, with some minor problems, also in DOS using HX DOS Extender. A port for DOS using DJGPP is also available. A version for the Wii Homebrew Channel has also emerged. It has also been ported to run on the Amazon Kindle.

MPlayer supports a wide variety of media formats and can also save all streamed content to a file.

A companion program, MEncoder, can take an input stream or file and transcode it into several different output formats, optionally applying various transforms along the way.

Development of MPlayer began in 2000. The original author, Árpád Gereöffy (known as A'rpi / Astral in the demoscene), was soon joined by many other programmers. The project was started because A'rpi was unable to find any satisfactory video players for Linux after XAnim stopped development in 1999. The first version was titled mpg12play v0.1 and was hacked together in a half hour using libmpeg3 from http://www.heroinewarrior.com/. After mpg12play v0.95pre5, the code was merged with an AVI player based on avifile's Win32 DLL loader to form MPlayer v0.3 in November 2000. In the beginning most developers were from Hungary, but currently the developers are located worldwide. Alex Beregszászi has maintained MPlayer since 2003 when Árpád Gereöffy left MPlayer development to begin work on a second generation MPlayer. The MPlayer G2 project is currently abandoned, and all the development effort is put on MPlayer 1.0.