William M. Smith (May 23, 1827 – March 25, 1886) was an American politician who served as the 24th Speaker of the Illinois House of Representatives.

He was involved in Republican politics since the founding of the party and was unafraid to espouse his anti-slavery beliefs.

He was a delegate at the Illinois State Republican convention in 1860 and nominated Abraham Lincoln for the presidency.

He fiercely opposed the policies of Andrew Johnson,[4] and was known to Illinois newspapers as "The Duke of Lexington" due to his strong personality.

After Shelby M. Cullom was elected Governor of Illinois he appointed Smith to the board of the Railroad and Warehouse Commission, where he remained until ill health compelled him to resign.