William Smyth (January 3, 1824 – September 30, 1870) was a native of County Tyrone in Northern Ireland who became a politician, lawyer and judge in the United States during the nineteenth century.

Born in Eden, County Tyrone, Ireland on January 3, 1824, Smyth attended rural schools as a child, completed preparatory studies and immigrated to the United States with his parents in 1838.

While there, he attended the University of Iowa, where he studied law, and was subsequently admitted to the bar in 1847.

Following the completion of his law studies, Smyth opened a legal practice in Marion, Iowa.

During the Civil War, he served for two years as colonel of the 31st Iowa Volunteer Infantry Regiment.