The siege of Vicksburg (May 18 – July 4, 1863) was the final major military action in the Vicksburg campaign of the American Civil War. In a series of maneuvers, Union Major General Ulysses S. Grant and his Army of the Tennessee crossed the Mississippi River and drove the Confederate Army of Mississippi, led by Lieutenant General John C. Pemberton, into the defensive lines surrounding the fortress city of Vicksburg, Mississippi, leading to the successful siege and Confederate surrender.

Vicksburg was the last major Confederate stronghold on the Mississippi River; therefore, capturing it completed the second part of the Northern strategy, the Anaconda Plan. When two major assaults against the Confederate fortifications, on May 19 and 22, were repulsed with heavy casualties, Grant decided to besiege the city beginning on May 25. After holding out for more than 40 days, with their supplies nearly gone, the garrison surrendered on July 4. The Vicksburg campaign's successful ending significantly degraded the Confederacy's ability to maintain its war effort. This action, combined with a siege that led to the surrender of the downriver Port Hudson to Major General Nathaniel P. Banks on July 9, yielded command of the Mississippi River to the Union forces, that held it for the rest of the conflict.

The Confederate surrender on July 4, 1863 is sometimes considered, combined with General Robert E. Lee's defeat at Gettysburg by Major General George Meade the previous day, the war's turning point. It cut off the Trans-Mississippi Department (containing the states of Arkansas, Texas and part of Louisiana) from the rest of the Confederate States, effectively splitting the Confederacy in two for the rest of the war. Lincoln called Vicksburg "the key to the war."

After crossing the Mississippi River south of Vicksburg at Bruinsburg and driving northeast, Grant won battles at Port Gibson and Raymond and captured Jackson, the Mississippi state capital, on May 14, 1863, forcing Pemberton to withdraw westward. Attempts to stop the Union advance at Champion Hill and Big Black River Bridge were unsuccessful. Pemberton knew that the corps under Maj. Gen. William T. Sherman was preparing to flank him from the north, and so had no choice but to withdraw or be outflanked. Pemberton burned the bridges over the Big Black River and devastated the countryside as he retreated to the heavily defended city of Vicksburg.

The Confederates evacuated Hayne's Bluff, which was subsequently occupied by Sherman's cavalry on May 19, and Union steamboats no longer had to run the guns of Vicksburg, now being able to dock by the dozens up the Yazoo River. Grant could now receive supplies more directly than by the previous route, which ran through Louisiana, over the river crossing at Grand Gulf and Bruinsburg, then back up north.

Over half of Pemberton's army had been lost in the two preceding battles and many in Vicksburg expected General Joseph E. Johnston, in command of the Confederate Department of the West, to relieve the city—which he never did. Large numbers of Union troops were on the march to invest the city. They repaired the bridges over the Big Black River and crossed on May 18. Johnston urged his general, Pemberton, to abandon the city in order to save his troops—a demand Pemberton refused. Pemberton, a Northerner by birth, was probably influenced by his fear of public condemnation if he abandoned Vicksburg.

Pemberton, trying to please Jefferson Davis, who insisted that Vicksburg and Port Hudson must be held, and to please Johnston, who thought both places worthless militarily, had been caught in the middle, a victim of a convoluted command system and his own indecisiveness. Too dispirited to think clearly, he chose to back his bedraggled army into Vicksburg rather than evacuate the city and head north where he might have escaped to campaign again. When he chose to take his army into Vicksburg, Pemberton sealed the fate of his troops and the city he had been determined to defend.

— Vicksburg, Michael B. Ballard.