The Battle of Chickasaw Bayou, also called the Battle of Walnut Hills,fought December 26–29, 1862, was the opening engagement of the Vicksburg Campaign during the American Civil War. Confederate forces under Lt. Gen. John C. Pemberton repulsed an advance by Union Maj. Gen. William T. Sherman that was intended to lead to the capture of Vicksburg, Mississippi.

On December 26, three Union divisions under Sherman disembarked at Johnson's Plantation on the Yazoo River to approach the Vicksburg defenses from the northeast while a fourth landed farther upstream on December 27. On December 27, the Union army pushed their lines forward through the swamps toward the Walnut Hills, which were strongly defended. On December 28, several futile attempts were made to get around these defenses. On December 29, Sherman ordered a frontal assault, which was repulsed with heavy casualties, and then withdrew. This Confederate victory frustrated Maj. Gen. Ulysses S. Grant's attempts to take Vicksburg by a direct approach.

During this period, the overland half of Grant's offensive was also failing. His lines of communication were disrupted by raids by Brig. Gen. Nathan Bedford Forrest and by Maj. Gen. Earl Van Dorn, who destroyed a large supply depot in the Holly Springs Raid on December 20. Unable to sustain his army without these supplies, Grant abandoned his overland advance. Sherman realized that his corps would not be reinforced by Grant and decided to withdraw his expedition, moving to the mouth of the Yazoo on January 2. On January 5, Sherman sent a letter to General-in-Chief Henry W. Halleck, summing up the campaign, "I reached Vicksburg at the time appointed, landed, assaulted, and failed." He and his command were then temporarily assigned to Maj. Gen. John A. McClernand for an expedition up the Arkansas River and the Battle of Arkansas Post. Although Grant tried a number of operations, or "experiments", to reach Vicksburg over the winter, the Vicksburg Campaign did not begin again in earnest until April 1863.