William Worth Belknap (September 22, 1829 – October 12, 1890) was a lawyer, Union Army officer, government administrator in Iowa, and the 30th United States secretary of war, serving under President Ulysses S. Grant. Belknap was impeached on March 2, 1876, for his role in the trader post scandal but was acquitted by the Senate. Belknap was the first cabinet secretary in U.S. history to be impeached.
A native of New York, Belknap graduated from the College of New Jersey, now Princeton University, in 1848, studied law with a Georgetown attorney, and passed the bar in 1851. He moved to Iowa, where he practiced law in partnership with Ralph P. Lowe. Belknap entered politics as a Democrat and served one term in the Iowa House of Representatives.