William Emery Merrill (1837–1891) was an American soldier and military engineer. He was born at Fort Howard, Wisconsin, USA, graduated first in his class at West Point in 1859, and from September, 1860, to July, 1861, was assistant professor of engineering there.
In the Civil War, he served as assistant engineer in the Army of the Potomac during the Peninsular campaign and in the northern Virginia campaign, and from July, 1864, to September, 1865, commanded as colonel, a regiment of veteran volunteer engineers. During the war he received the successive brevets of captain, major, lieutenant colonel, and colonel for gallant services.
In March, 1867, he was raised to the regular rank of major and in February, 1883, to that of lieutenant colonel. From 1867 to 1870, he was chief engineer on the staff of General Sherman, then commanding the Military Division of the Missouri, and thereafter until his death he was engaged on engineering work for the government. One of the most notable engineering works of its kind in America was the Chauvine wicket movable dam constructed by him at Davis's Island, 5½ miles below Pittsburgh. In 1889, he represented the United States Engineering Corps at the International Congress of Engineers in Paris.
He published Iron Truss Bridges for Railroads (1870) and Improvement of Non-Tidal Rivers (1881).