A widget toolkit, widget library, GUI toolkit, or UX library is a library or a collection of libraries containing a set of graphical control elements (called widgets) used to construct the graphical user interface (GUI) of programs.

Most widget toolkits additionally include their own rendering engine. This engine can be specific to a certain operating system or windowing system or contain back-ends to interface with more multiple ones and also with rendering APIs such as OpenGL, OpenVG, or EGL. The look and feel of the graphical control elements can be hard-coded or decoupled, allowing the graphical control elements to be themed/skinned.