On 7 April 1943 she departed for the Aleutian Islands, Alaska, for pre and post-invasion bombardment missions. Serving as flagship she shelled Kiska Island from June to mid August. On one occasion she and her sister Mississippi engaged in a 'ship-to-ship battle.' Both ships were relying on radar alone since the visibility was the typical - near zero. About 80 miles west of Kiska, radar contacts were detected and the ships opened fire with their main batteries. But it was a decidedly one sided affair since there were no Japanese warships within 200 miles of them. This unique encounter earned the name the "Battle of the Pips."

With the Aleutians recaptured, she returned to San Francisco 7 September 1943 to prepare for her next mission. As a part of the Northern Attack Force (Task Group 52.2) they began bombarding Makin Island on 20 November. She continued to provide support until 5 December when she departed for Pearl Harbor. The task group then sailed to the Marshall Islands in January 1944 and she began bombarding Kwajalein on 31 January until 5 February. On 20 March 1944 four battleships, including Idaho, shelled Kavieng, New Ireland, as a diversion from the Army invasion of Emirau Island.

On 8 June 1944, Idaho arrived at Kwajalein with several escort carriers to sail to the Marianas Islands. On 14 June she commenced bombarding Saipan and supported the landings the next day. Between 19 to 21 June she protected convoys and transports during the Battle of the Philippine Sea. During the end of the month and the beginning of July she returned to Eniwetok. She then sailed to Guam on 12 July for more bombardment and continued to provide fire support until returning to Eniwetok on 2 August 1944.

Idaho underwent repairs in a floating drydock at Espirtu Strait on 15 August 1944 and then supported the landings in the Palau Islands between 12 through 24 September. She departed the warzone for Puget Sound Navy Yard for another overhaul. Here she was refitted again between 22 October 1944 through 1 January 1945 and her 5in/25 cal dual purpose guns were removed and replaced with ten single enclosed 5in/38cal turrets. Unique among all US battleships, she was the only one to be equipped with the single mount 5in/38cal DP guns. These single mounts were usually carried on destroyers while battleships usually carried the twin mounts. Following her refit she operated briefly off the coast before returning to Pearl Harbor on 20 January.

In mid February 1945 Idaho bombarded and supported the landings at Iwo Jima until departing for Ulithi on 7 March to prepare for Okinawa. Along with battleship West Virginia, she was a part of Unit 4 in Task Force 54 and joined in the shelling of Okinawa on 25 March. During the operation, waves of kamikazes attacked the fleet and Idaho was successful in shooting down five aircraft but in the process sustained damage on her port side from a near miss. She sailed to Guam on 20 April for repairs and returned to Okinawa on 22 May to resume her fire support role. She departed on 20 June for Leyte Gulf to prepare for the invasion of the Japanese home islands and was there when the war ended on 20 August. After this she performed occupational duties she entered Toyko Bay as a member of Halsey's Task Force 38 and was able to witness the surrender on 2 September.

With the war officially over, Idaho departed for Norfolk Navy Yard, Virginia, in October 1945. She was decommissioned on 3 July 1946 and sold for scrap to Lipsett Inc of New York on 24 November 1947. She was awarded seven battle stars for her service in World War II.