Ostend Manifesto, in United States history, a dispatch drafted in Ostend, Belgium, October, 1854, by three United States diplomats and sent to Secretary of State William L. Marcy. The manifesto involved Cuba, then a possession of Spain. Marcy had instructed Pierre Soulé, minister to Spain, to try to buy Cuba but his efforts were unsuccessful. Soulé was then instructed to consult with James Buchanan, minister to Great Britain, and John Y. Mason, minister to France. Soulé and Mason, both Southerners, hoped to make Cuba into a slave state; Buchanan, although a Northerner, was sympathetic to the South.

The manifesto referred to a fear that Cuba might come under the control of Cuban blacks. The three diplomats stated that if Spain should refuse to sell the island, the United States might consider taking it by force. Although intended to be secret, the contents of the manifesto became known to the public and led to strong criticism at home and abroad. Marcy repudiated it.