The Third Treaty of San Ildefonso was a secret agreement signed on 1 October 1800 between Spain and the French Republic by which Spain agreed in principle to exchange its North American colony of Louisiana for territories in Tuscany. The terms were later confirmed by the March 1801 Treaty of Aranjuez.

The Convention of 1800, also known as the Treaty of Mortefontaine, was signed on September 30, 1800, by the United States and France. The difference in name was due to Congressional sensitivity at entering into treaties, due to disputes over the 1778 treaties of Alliance and Commerce between France and the U.S.