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 between the United States and Great Britain in an aggressive measure to end the Atlantic slave trade. It was negotiated by U.S. Secretary of State William H. Seward and British Ambassador to the U.S. Richard Lyons, 1st Viscount Lyons. The treaty was concluded in Washington, on April 7, 1862, and was unanimously ratified by the U.S. Senate on April 25, 1862. Ratifications were exchanged in London, on May 25, 1862.