William P. Fessenden, American lawyer and politician, 26th United States Secretary of the Treasury (d. 1869)
William Pitt Fessenden (October 16, 1806 – September 8, 1869) was an American politician from the U.S. state of Maine. Fessenden was a Whig (later a Republican) and member of the Fessenden political family. He served in the United States House of Representatives and Senate before becoming Secretary of the Treasury under President Abraham Lincoln during the American Civil War. Fessenden then re-entered the Senate, where he died in office in 1869.

A lawyer, he was a leading antislavery Whig in Maine; in Congress, he fought the Slave Power, plantation owners who controlled Southern states. He built an antislavery coalition in the state legislature that elected him to the U.S. Senate; it became Maine's Republican organization. In the Senate, Fessenden played a central role in the debates on Kansas, denouncing the expansion of slavery. He led Radical Republicans in attacking Democrats Stephen Douglas, Franklin Pierce, and James Buchanan. Fessenden's speeches were read widely, influencing Republicans such as Abraham Lincoln and building support for Lincoln's 1860 Republican presidential nomination. During the war, Senator Fessenden helped shape the Union's taxation and financial policies. He abandoned his earlier radicalism, joining pro-Lincoln Moderate Republicans against the Radicals and becoming Lincoln's Treasury Secretary.After the war, Fessenden was back in the Senate, as chair of the Joint Committee on Reconstruction, which established terms for resuming congressional representation for the southern states, and which drafted the Fourteenth Amendment to the United States Constitution. Later, Fessenden provided critical support that prevented the Senate conviction of President Andrew Johnson, who had been impeached by the House. He was the first Republican senator to ring out "...not guilty" followed by six other Republican senators, ultimately resulting in the acquittal of President Johnson. Fessenden's vote against convicting Johnson were motivated by his support for free trade and fears of a Benjamin Wade presidency.He is the only person to have three streets in Portland named for him: William, Pitt and Fessenden streets in the city's Oakdale neighborhood.