The Ostend Manifesto, also known as the Ostend Circular, was a document written in 1854 that described the rationale for the United States to purchase Cuba from Spain while implying that the U.S. should declare war if Spain refused. Cuba's annexation had long been a goal of U.S. slaveholding expansionists. At the national level, American leaders had been satisfied to have the island remain in weak Spanish hands so long as it did not pass to a stronger power such as Britain or France. The Ostend Manifesto proposed a shift in foreign policy, justifying the use of force to seize Cuba in the name of national security. It resulted from debates over slavery in the United States, manifest destiny, and the Monroe Doctrine, as slaveholders sought new territory for the expansion of slavery.

During the administration of President Franklin Pierce, a pro-Southern Democrat, Southern expansionists called for acquiring Cuba as a slave state, but the outbreak of violence following the Kansas–Nebraska Act left the administration unsure of how to proceed. At the suggestion of Secretary of State William L. Marcy, American ministers in Europe—Pierre Soulé for Spain, James Buchanan for Britain, and John Y. Mason for France—met to discuss strategy related to an acquisition of Cuba. They met secretly at Ostend, Belgium, and drafted a dispatch at Aachen, Prussia. The document was sent to Washington in October 1854, outlining why a purchase of Cuba would be beneficial to each of the nations and declaring that the U.S. would be "justified in wresting" the island from Spanish hands if Spain refused to sell. To Marcy's chagrin, Soulé made no secret of the meetings, causing unwanted publicity in both Europe and the U.S. The administration was finally forced to publish the contents of the dispatch, which caused it irreparable damage.

The dispatch was published as demanded by the House of Representatives. Dubbed the "Ostend Manifesto", it was immediately denounced in both the Northern states and Europe. The Pierce administration suffered a significant setback, and the manifesto became a rallying cry for anti-slavery Northerners. The question of Cuba's annexation was effectively set aside until the late 19th century, when support grew for Cuban independence from Spain.