A RIDL file contains type library data referenced by projects created by Embarcadero RAD Studio, a program used to design user interfaces (UIs) for Windows, macOS, iOS, and Android apps. It stores various information about objects, interfaces, object classes, and member functions. RIDL files are saved in the Restricted Interface Definition Language (RIDL) format.

You will most likely only encounter RIDL files if you use RAD Studio for developing app UIs. Developers typically create and edit the files with RAD Studio, although the files can also be opened with a text editor since they are saved in plain text. RIDL files were introduced in 2009 by Embarcadero as a replacement for libraries saved in the Interface Definition Language (IDL) format. The RIDL format provides more editing flexibility. The files are compiled by the GenTLB.exe type library generator (included with RAD Studio) into binary TLB files when the project is built.