The 90th Infantry Regiment was a Regular Army infantry regiment of the United States Army, which existed during World War I and World War II. The regiment was organized in 1918 during World War I with the 20th Division, but the war ended before it could be deployed overseas; it was demobilized in the northern hemisphere spring of 1919. During World War II, the 90th Infantry was again activated with the 10th Light Division in mid-1943, but was transferred to become a nondivisional separate training unit in early 1944, inactivating in mid-1945.
Following the Attack on Pearl Harbor, which resulted in the United States entry into World War II, the regiment was reconstituted on 10 July 1943, and activated five days later at Camp Hale, Colorado with the 10th Light Division, using a cadre from the 105th, 106th, and 165th Infantry Regiments of the 27th Infantry Division, then in Hawaii. These men were reinforced by skiers and mountain climbers from the 86th Infantry Regiment, and the regiment conducted ski and mountain warfare training for the next several months. It was successively commanded by Colonels James E. Lewis and Paul R. Goode. It was relieved from its assignment to the 10th on 13 or 22 February 1944, and transferred to XVI Corps at Camp Carson as a separate unit. Most of its skiers from Camp Hale were used to reinforce the other regiments of the division before the transfer; they were replaced by men from the 87th Infantry Regiment and other units of the 10th. At Camp Carson, the 611-man cadre of the regiment provided an accelerated six-week course of infantry training to 2,900 men, who were formerly members of disbanded anti-aircraft and tank destroyer units or who had volunteered for transfer to the infantry from other branches of the Army, under the command of Colonel James E. Graham. Graham was replaced by Colonel Winfield R. McKay on 26 August. After XVI Corps was sent to Europe the 90th was assigned to XXXVI Corps on 17 July before it moved to Camp Gruber, Oklahoma on 1 December. There, the regiment joined the Replacement and School Command on 9 February 1945, relocating to Camp Rucker, Alabama, on 17 February, where it inactivated on 10 August after training replacements for the remainder of the war. The regiment published a newspaper, titled The 90th Infantryman, beginning at Camp Carson. The 90th Infantry was never authorized a distinctive unit insignia or coat of arms.