The Treaty between the United States and Great Britain for the Suppression of the Slave Trade, also known as the Lyons-Seward Treaty, was a treaty entered into between the United States and the United Kingdom. It was negotiated by U.S. Secretary of State William H. Seward and British Ambassador to the United States Richard Lyons, 1st Viscount Lyons. The treaty was concluded in Washington, D.C. on April 7, 1862, and was unanimously ratified by the United States Senate on April 25, 1862. Ratifications were exchanged in London, on May 25, 1862. It was proclaimed by President Abraham Lincoln on June 7, 1862.
The treaty set forth aggressive measures to end the Atlantic slave trade, including an agreement that the respective countries would use their navies to seize merchant vessels carrying captured Africans, including any vessel bearing indications of being a slave trading vessel, such as grated hatches instead of closed hatches, stores of food and water far exceeding the needs of a normal crew, and shackles or chains. It conceded to England the right of search to a limited extent in African and Cuban waters, but secured a similar concession for American war vessels from the British government. Seward, by his course in the Trent Affair which preceded the negotiation of the Treaty, virtually committed Great Britain to the American attitude with regard to this right.