A graphical user interface builder (or GUI builder), also known as GUI designer or sometimes RAD IDE, is a software development tool that simplifies the creation of GUIs by allowing the designer to arrange graphical control elements (often called widgets) using a drag-and-drop WYSIWYG editor. Without a GUI builder, a GUI must be built by manually specifying each widget's parameters in the source code, with no visual feedback until the program is run. Such tools are usually called the term RAD IDE.

User interfaces are commonly programmed using an event-driven architecture, so GUI builders also simplify creating event-driven code. This supporting code connects software widgets with the outgoing and incoming events that trigger the functions providing the application logic.

Some graphical user interface builders automatically generate all the source code for a graphical control element. Others, like Interface Builder or Glade Interface Designer, generate serialized object instances that are then loaded by the application.