William P. Fessenden, American lawyer and politician, 26th United States Secretary of the Treasury (died 1869)

William Pitt Fessenden was a politician from Maine, United States. He was a Whig 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.

The United States secretary of the treasury is the head of the United States Department of the Treasury, and is the chief financial officer of the federal government of the United States. The secretary of the treasury serves as the principal advisor to the president of the United States on all matters pertaining to economic and fiscal policy. The secretary is, by custom, a member of the president's cabinet and, by law, a member of the National Security Council, and fifth in the U.S. presidential line of succession.