Assiento, or Asiento, Treaty, a Spanish term signifying a contract or convention entered into between Spain and some other country for regulating the supply of negroes for its American colonies. Spain having little intercourse with those parts of Africa from which slaves were obtained, used to contract with some other nation that had establishments on the western coast of that continent for the supply of its South American possessions with negroes. Such treaties were made first with Portugal and afterwards with France, each of which countries in consideration of enjoying a monopoly of the supply of negroes to the South American dominions of Spain, agreed to pay to that Crown a certain sum for each negro imported. In both cases the assiento was taken by a commercial association in France. Both the Portuguese Company and the French were ruined by their contract. England was a party to a similar treaty during the twenty-six years which preceded the Treaty of Madrid in 1750.