Sundown towns, also known as sunset towns, gray towns, or sundowner towns, are all-white municipalities or neighborhoods in the United States and Canada that were most prevalent before the mid-20th century, which practiced a form of racial segregation by excluding non-whites via some combination of discriminatory local laws, intimidation or violence. The term came into use because of signs that directed "colored people" to leave town by sundown.

Sunset is the disappearance of the Sun below the horizon of the Earth due to its rotation. As viewed from everywhere on Earth, it is a phenomenon that happens approximately once every 24 hours, except in areas close to the poles. The equinox Sun sets due west at the moment of both the spring and autumn equinoxes. As viewed from the Northern Hemisphere, the Sun sets to the northwest in the spring and summer, and to the southwest in the autumn and winter; these seasons are reversed for the Southern Hemisphere.