The secondary battery consisted of fourteen /40 caliber Mark IV guns, which were placed in casemates in the hull. They fired a shell at a muzzle velocity of . For close-range defense against torpedo boats, they carried sixteen 6-pounder guns mounted in casemates along the side of the hull and six 1-pounder guns. These guns fired and shells, respectively. As was standard for capital ships of the period, the Illinois class carried four torpedo tubes in above-water, hull mounted torpedo launchers. They were initially equipped with the Mark II Whitehead design, which carried a warhead and had a range of at a speed of .
