William Worth Belknap (September 22, 1829 - October 13, 1890) was a U.S. administrator, soldier and United States Secretary of War.

Belknap was born in Newburgh, New York. He graduated from Princeton University in 1848 and studied law at Georgetown University. In 1851 Belknap was admitted to the bar, moved to Keokuk, Iowa and entered the practice of law. He served in the state legislature from 1857 to 1858.

In 1861 he was commissioned major in the 15th Iowa Infantry and participated in the Civil War battles of Shiloh, Corinth , and Vicksburg In 1864 Belknap was promoted to brigadier general and given command of the 4th Division, XVII Corps and participated in General Sherman's operations in Georgia and the Carolinas. He was mustered out of service as a major general in 1865.

He married and lost his first two wives, Cora LeRoy and Carrie Tomlinson, and married Mrs. John Bower, his second wife's sister. From 1865 to 1869 Belknap was collector of internal revenue in Iowa. From October 25 1869 to March 2 1876 he served as Secretary of War.

Belknap recommended that Congress act to fix May 1 as the start of the fiscal year, inaugurated the preparation of historical reports by post commanders and proposed actions to preserve Yellowstone National Park. He was impeached by a unanimous vote of the House of Representatives for allegedly having received money in return for post tradership appointments. He resigned the secretaryship before being brought to trial in March 1876, was tried by the United States Senate, but the vote fell short of the two-thirds required for conviction.

Belknap moved to Philadelphia, then returned to Washington to resume the practice of law. He died in Washington, D.C., on October 13 1890.