R file is a text file containg source code used by Rez resource compiler. Rez is a part of Apple Xcode package available on Mac Os X operating system.

R file contains application resource daclarations regarding, for example, alerts, icons, and othres data used for generating application resource files. For R file to be compiled, it must contain not only type declarations, but also resource definitions. Compiled file can be decompiled into R file and then reused.