The Ostend Manifesto was a secret document written in 1854 by U.S. diplomats at Ostend, Belgium, describing a plan to acquire Cuba from Spain.

On orders from U.S. Secretary of State William L. Marcy, three U.S. diplomats—minister to Britain James Buchanan, minister to France John Y. Mason, and minister to Spain Pierre Soulé—devised a plan to purchase Cuba, for $120 million, for the United States. Further, if Spain were to the refuse the offer, the manifesto suggested that America would be "justified in wresting" Cuba from Spain. The document was then sent back to the U.S. State Department, but news of it leaked out and it was soon made public.

The aggressively worded document, and Soulé's advocacy of slavery, caused outrage among Northerners who felt it was a Southern attempt to extend slavery. American free-soilers, just recently stirred with the Fugitive Slave Law passed as part of the Compromise of 1850, decried the "manifesto of brigands." Thus the American scheme to capture Cuba fizzled.

American intervention in Cuba would next surface near the end of the nineteenth century in the Spanish-American War.