William Duncan Smith (July 28, 1825 – October 4, 1862) was a career United States Army officer who fought in the Mexican–American War.

He entered the United States Military Academy in West Point in July 1842, and graduated four years later, standing 35th out of 59 cadets.

[1] Smith served during the Mexican-American War, and was wounded in the Battle of Molino del Rey on September 8, 1847, one of the conflict's bloodiest engagements.

[2] Choosing to follow his home state and the Confederate cause, Smith resigned his U.S. commission on January 28, 1861.

He entered service in the Confederate States Army on March 16 as a captain in the cavalry, but transferred to the infantry that same day, also with a promotion to major.