Plaçage was a recognized extralegal system in French and Spanish slave colonies of North America (including the Caribbean) by which ethnic European men entered into civil unions with non-Europeans of African, Native American and mixed-race descent. The term comes from the French placer meaning "to place with". The women were not legally recognized as wives but were known as placées; their relationships were recognized among the free people of color as mariages de la main gauche or left-handed marriages. They became institutionalized with contracts or negotiations that settled property on the woman and her children and, in some cases, gave them freedom if they were enslaved. The system flourished throughout the French and Spanish colonial periods, reaching its zenith during the latter, between 1769 and 1803.

It was widely practiced in New Orleans, where planter society had created enough wealth to support the system. It also took place in the Latin-influenced cities of Natchez and Biloxi, Mississippi; Mobile, Alabama; St. Augustine and Pensacola, Florida; as well as Saint-Domingue (now the Republic of Haiti). Plaçage became associated with New Orleans as part of its cosmopolitan society.

Citing the lack of any extant contracts or other records, at least three historians (viz. Kenneth Aslakson, Emily Clark, and Carol Schlueter) have challenged the historicity of plaçage and have referred to many of its features, including quadroon balls, as "a myth".