Army of the Mississippi was the name given to two Union armies during the American Civil War.^[1] Both operated around the Mississippi River and both were active for only a short time.

The first army was created on February 23, 1862.^[2] It was commanded by Major general John Pope.^[2] Pope's orders were to clear Confederate troops along the Mississippi River so the Union army could move through the area.^[3] On February 28, 1862, General Pope began to move against New Madrid, Missouri.^[4] The Army then fought at the Battle of Island Number Ten.^[3] The army next fought at the Siege of Corinth. After the capture of Corinth Pope was sent east to command the Army of Virginia. Major general William S. Rosecrans assumed command of the Army of the Mississippi. The army fought at the Battle of Iuka followed by the Second Battle of Corinth. His commanding officer, Ulysses S. Grant, was unhappy with his failure to go after the retreating Confederates after the Battle of Corinth.^[5] In October, 1862, Rosecrans was transferred to command the Army of the Ohio and the Army of the Mississippi was disbanded.

In 1863 Major general John Alexander McClernand was put in command of the expedition against the Confederates down the Mississippi. He was given command of two corps from the Army of the Tennessee. McClernand named his force the "Army of the Mississippi". McClernand was successful in capturing Arkansas Post but he acted without orders.^[6] On January 30, 1863, Grant personally took command of the army.^[6] The old XIII Corps and XV Corps were returned to the Army of the Tennessee on January 12, 1863.