William Barrett Washburn (January 31, 1820 – October 5, 1887) was an American businessman and politician from Massachusetts. Washburn served several terms in the United States House of Representatives (1863–71) and as the 28th Governor of Massachusetts from 1872 to 1874, when he won election to the United States Senate in a special election to succeed the recently deceased Charles Sumner. A moderate Republican, Washburn only partially supported the Radical Republican agenda during the American Civil War and the Reconstruction Era that followed.