In early 1863, the regiment was transferred to the Big Black River, where it guarded a bridge. In March, the regiment moved to Grand Gulf, Mississippi, where the men built fortifications, strengthening the defenses of Vicksburg, Mississippi. On April 5, the 3rd Missouri Infantry, as part of a larger force, was moved across the Mississippi River into Louisiana, where they encamped near Bayou Vidal in Tensas Parish. On April 8, the unit fought in a small action at James' Plantation nearby. An attempt by the reconnaissance force to strike a major blow against Union forces occupying the area on April 15 failed when the element of surprise for a planned attack was lost. On April 17, the arrival of Union Navy ships forced the Confederates to return to Mississippi. The regiment then rejoined the defenses at Grand Gulf. At the Battle of Grand Gulf on April 29, the 3rd Missouri Infantry defended a line of rifle pits that extended between the two main Confederate fortifications. The unit's position allowed the men to shoot into the portholes of the Union Navy ships shelling the Confederate position. The regiment lost one man killed and three wounded in the battle. The Confederate victory prevented Union Major General Ulysses S. Grant from landing a force at that point, although Grant was able to land a strong force south of the position. Brigadier General John S. Bowen, the Confederate commander at Grand Gulf, responded to the Union landing by sending a force to Port Gibson, Mississippi to intercept Grant.