The 33rd Illinois transferred to the 1st Brigade, 14th Division, XIII Corps, Department of the Tennessee during the period March–July 1863. The regiment was ordered to Ste. Genevieve, Missouri, on 5 March, and was then shipped to Milliken's Bend near Vicksburg, Mississippi, where it remained until 25 April. The unit began its participation in the Vicksburg campaign when it moved to Bruinsburg, Mississippi, on 25–30 April. During the campaign, the 33rd Illinois was in Benton's brigade together with the 99th Illinois, 8th Indiana, and 18th Indiana Infantry Regiments. Benton's brigade was part of Brigadier General Eugene Asa Carr's division, Major General John Alexander McClernand's corps, and Major General Ulysses S. Grant's army. The 33rd Illinois fought at the Battle of Port Gibson on 1 May, Battle of Champion Hill on 16 May, Battle of Big Black River Bridge on 17 May, and Siege of Vicksburg on 18 May–4 July.