In computing, a modifier key is a special key (or combination) on a computer keyboard that temporarily modifies the normal action of another key when pressed together. By themselves, modifier keys usually do nothing; that is, pressing any of the ⇧ Shift, Alt, or Ctrl keys alone does not (generally) trigger any action from the computer.

For example, in most keyboard layouts the Shift key combination ⇧ Shift+A will produce a capital letter "A" instead of the default lower-case letter "a" (unless in Caps lock or Shift lock mode). A combination of Alt+F4 in Microsoft Windows will close the active window; in this instance, Alt is the modifier key. In contrast, pressing just ⇧ Shift or Alt will probably do nothing unless assigned a specific function in a particular program (for example, activating input aids or the toolbar of the active window in Windows).

User interface expert Jef Raskin coined the term "quasimode" to describe the state a computer enters into when a modifier key is pressed.