Sundown towns, also known as sunset towns, gray towns, or sundowner towns, were all-White municipalities or neighborhoods in the United States that practiced a form of racial segregation by excluding non-whites via some combination of discriminatory local laws, intimidation or violence. They were most prevalent before the 1950s. The term came into use because of signs that directed "colored people" to leave town by sundown.

Sundown counties and sundown suburbs were created as well. While sundown laws became illegal following the passage of the Civil Rights Act of 1968, some commentators hold that certain 21st-century practices perpetuate a modified version of the sundown town. Some of these modern practices include racial profiling by local police and sheriff's departments, vandalism of public art, harassment by private citizens, and gentrification.

Specific examples of exclusion of Native Americans, Asians, Latinos, Jews, and Catholics alongside many other communities include towns such as Minden and Gardnerville, Nevada, in which sirens were used from 1917 until 1974 to signal Native Americans to leave town by 6:30 p.m. each evening, a practice that symbolically persisted into the 21st century. In Antioch, California, Chinese residents faced curfews as early as 1851, and in 1876, a mob destroyed the Chinatown district, prompting a mass exodus that left only a small number of Chinese residents by the mid-20th century. Mexican Americans were excluded from Midwestern sundown towns through racially restrictive housing covenants, signs (often posted within the same infamous "No Blacks, No Dogs" signs), and police harassment. Additionally, Jews and Catholics were unwelcome in certain communities, with some towns explicitly prohibiting them from owning property or joining local clubs.

Black Americans were also impacted through widespread and often well-documented exclusionary policies. These discriminatory policies and actions distinguish sundown towns from towns that have no Black residents for demographic reasons. Historically, towns have been confirmed as sundown towns by newspaper articles, county histories, and Works Progress Administration files; this information has been corroborated by tax or U.S. census records showing an absence of Black people or a sharp drop in the Black population between two censuses.

The earliest legal restrictions on the nighttime activities and movements of African Americans and other racial minorities date back to the colonial era. The general court and legislative assembly of the Province of New Hampshire passed "An Act to Prevent Disorders in the Night" in 1714:

Whereas great disorders, insolencies and burglaries are oft times raised and committed in the night time by Indian, Negro, and Molatto Servants and Slaves to the Disquiet and hurt of her Majesty's subjects, No Indian, Negro, or Molatto is to be from Home after 9 o'clock.

Notices emphasizing and re-affirming the curfew were published in The New Hampshire Gazette in 1764 and 1771. Following the American Revolutionary War, Virginia was the first U.S. state to prohibit the entry of all Free Negros. In the Northwest Territory, which existed from 1787 to 1803, local authorities across the region imposed special regulations on free Black Americans migrating to the territory, known as "black laws"; these included requiring them to register themselves with county officials and post bonds "designed to ensure that they would not become dependent on public relief." As noted by the historian Kate Masur, "many Americans–in free states and slave states alike–viewed laws that imposed special regulations on free people of African or Native American descent as just another appropriate use of the police power."

Following the end of the Reconstruction era, thousands of towns and counties across the United States became sundown localities, as part of the imposition of Jim Crow laws and other segregationist practices. In most cases, the exclusion was official town policy or was promulgated by the community's real estate agents via exclusionary covenants governing who could buy or rent property. In others, the policy was enforced through intimidation. This intimidation could occur in several ways, including harassment by law enforcement officers. Though no sundown towns exist today in the sense of publicly or legally excluding non-white residents, some commentators have applied the term to towns practicing other forms of racial exclusion.