William Horace Clagett (September 21, 1838 – August 3, 1901) was a nineteenth-century politician and lawyer from various places in the United States.

Born in Upper Marlboro, Maryland, Clagett moved to Keokuk, Iowa with his father in 1850 where he attended the public schools as a child.

He was elected a Republican from the Montana Territory to the United States House of Representatives in 1870, serving from 1871 to 1873, being unsuccessful for reelection in 1872.

On December 18, 1871, at the urging of Ferdinand Vandeveer Hayden and after learning of the findings of the Hayden Geological Survey of 1871, Clagett introduced the Act of Dedication bill into the House that ultimately led to the creation of Yellowstone National Park.

Afterwards leaving Congress, Clagett resumed practicing law in Deer Lodge, Montana, Denver, Colorado, Deadwood, Dakota, Portland, Oregon and Coeur d'Alene, Idaho.