QTX is the file extension associated with the QuickTime Player multimedia playback program. QTX files are a plug-in to the program that allows you to extend its functionality. For correct operation, the QTX extension file must be located in the appropriate folder. QTX files allow you to play multimedia content encoded with other codecs than those that QuickTime Player could originally handle.

QuickTime is a tool developed and published by Apple for its operating systems as well as for Windows systems.