John S. Marmaduke (born John Sappington Marmaduke, March 14, 1833 – December 28, 1887) serves as the 25th governor of Missouri from 1885 until his death in 1887.

Prior to that, he is a senior officer of the Confederate States Army who commands cavalry in the Trans-Mississippi Theater of the American Civil War.

On September 6, 1863 Marmaduke kills Confederate Brigadier-General Lucius M. Walker in a duel.

Major-General Sterling Price orders Marmaduke's arrest, but suspends the order because of the impending Union advance on Little Rock, Arkansas.

Marmaduke will never face a court martial for the duel.