ResEdit was a developer tool application for the Apple Macintosh, used to create and edit resources directly in the Mac's resource fork architecture. It was an alternative to tools such as REdit, and the resource compiler Rez. For the average user, ResEdit was generally easier to use, because it used a graphical user interface. Although it had been intended to be a developer tool, power users often used it to edit icons, menus, and other elements of an application's GUI, customizing it to their own preferences.

Resources on the Macintosh could be of many different types, and in fact any arbitrary data could be turned into a resource. While the system defined many standard formats for particular kinds of resources (for example, an icon, or a window template), programmers were also free to define their own. ResEdit included support for editing many of the standard types and for creating arbitrary resources with any structure a programmer might have dreamed up.

ResEdit was one of the earliest examples of a GUI layout tool, an essential component for rapid application development. For example, the Mac OS defined a standard resource called a dialog template and a dialog items list (resource types 'DLOG' and 'DITL' respectively). In ResEdit, it was possible to simply create these types and add GUI elements to them in an almost WYSIWYG fashion, so that you could design a user interface directly as it would appear to the end user of your application. Later, the application code could create a functional dialog box using the stored resource data which would match the appearance you laid out in ResEdit. While hardly a revolutionary concept today, when ResEdit first appeared in the mid-1980s, this was a considerable innovation. ResEdit included standard editors for window templates ('WIND'), menus ('MENU'), dialog boxes, controls ('CNTL'), color palettes ('clut' and 'pltt'), icons ('ICON', 'cicn', 'ICN#'), and various other standard types.