Her main armament consisted of fourteen breech-loading (BL) 6-inch Mk VII guns. Four of these guns were mounted in two twin-gun turrets, one each fore and aft of the superstructure and the others positioned in casemates amidships. Six of these were mounted on the main deck and were only usable in calm weather. They had a maximum range of approximately with their shells. Ten quick-firing (QF) 12-pounder 12 cwt guns were fitted for defence against torpedo boats. "Monmouth" also carried three 3-pounder Hotchkiss guns and two submerged 18-inch torpedo tubes.