A RAR file is a proprietary file format for archiving and data compression. A Russian engineer named Eugene Roshal developed this type of file for MS-DOS before it was later used in other operating systems as well.

RAR files are very popular in a Windows specific version called WinRAR, which is an encoder that has been released as shareware. Roshal released a decoder's source code for free distribution and modification, with the explicit condition that users will never try to take his ideas and turn them into a similar product. This typically is not an issue anymore, since there are many comparable compression tools that one can be found such as 7-zip.

RAR files can be identified by the .rar extension, the MIME type being application /x-rar-compressed.