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.

Louisiana, or the Province of Louisiana, was a province of New Spain from 1762 to 1801 primarily located in the center of North America encompassing the western basin of the Mississippi River plus New Orleans. The area had originally been claimed and controlled by France, which had named it La Louisiane in honor of King Louis XIV in 1682. Spain secretly acquired the territory from France near the end of the Seven Years' War by the terms of the Treaty of Fontainebleau (1762). The actual transfer of authority was a slow process, and after Spain finally attempted to fully replace French authorities in New Orleans in 1767, French residents staged an uprising which the new Spanish colonial governor did not suppress until 1769. Spain also took possession of the trading post of St. Louis and all of Upper Louisiana in the late 1760s, though there was little Spanish presence in the wide expanses of what they called the "Illinois Country".