The 9th Infantry Regiment ("Manchu") is a parent^{[further explanation needed]} infantry regiment of the United States Army.

Unrelated units designated the 9th Infantry Regiment were organized in the United States Army in 1798 during the Quasi-War, in 1812 during the war of 1812, and in 1847 during the Mexican–American War. The 1812 regiment fought in the Battle of Lundy's Lane, and the 1847 regiment in the Battle for Mexico City.

The lineage of the current regiment begins with the 1855 organization of the 9th Infantry Regiment, which was dispatched to the Pacific Northwest, where it served in the American Indian Wars. The regiment remained in the west during the American Civil War, garrisoning posts near San Francisco. After the end of the American Civil War the regiment continued its service through the final Indian Wars, then fought at the Battle of San Juan Hill during the Spanish–American War. During the Boxer Rebellion, the 9th Infantry was sent to China, where it earned the nickname Manchu. After the end of the rebellion the regiment saw duty in the Philippine–American War.

In 1917 the regiment became part of the 2nd Infantry Division, with which it served during World War I, World War II, and the Korean War. Reorganized as a parent regiment during the late 1950s as the United States Army adapted its organization to the Cold War, its 4th Battalion served with the 25th Infantry Division in the Vietnam War. The 9th's 1st, 2nd, and 3rd Battalions served in the 1989–1990 United States invasion of Panama, Operation Just Cause, with the 7th Infantry Division (Light). Its 1st and 4th Battalions fought in the Iraq War and 4th Battalion later fought in Operation Enduring Freedom in Afghanistan in 2012-2013. Detachments of 4th Battalion deployed again to Afghanistan in support of Operation Freedom’s Sentinel in 2018-2019 and to Iraq and Syria as part of Operation Inherent Resolve in 2021-2022. 4th Battalion is, as of February 2018^[update], the only remaining active battalion of the regiment, stationed at Fort Carson with the 1st Stryker Brigade Combat Team, 4th Infantry Division.

The first unit designated “9th Infantry Regiment” was one of the first permanent units authorized for the United States Army. It first appeared as a result of the Act of Congress of 16 July 1798, that authorized twelve additional regiments of infantry, in January 1799. Josiah Carville Hall, of Maryland, was its lieutenant colonel. All of the officers were appointed from Maryland, and the regiment was recruited in that state. However it was disbanded 15 June 1800. It appeared again serving in the War of 1812, it was again organized in March 1812, with Simon Learned, of Massachusetts, as colonel. The regiment was raised in Massachusetts, and took part in the war on the northern border, being present at the Battle of Lundy's Lane, and other actions in that area. Following the war in the reorganization of the army, this regiment was again disbanded. These early units called the “9th Infantry” have no lineal connection to the present regiment that was formed in 1855.

As a result of the Mexican–American War, in April 1847, the 9th Infantry was again organized, as one of the ten one-year regiments authorized by the Act of 11 February 1847. It was recruited primarily from the six New England states. It was briefly commanded by Colonel (and future President of the United States) Franklin Pierce before Pierce was promoted to brigadier general and commander of the brigade that included the 9th Regiment. Pierce was succeeded by Colonel Truman B. Ransom, who was killed in the assault upon Chapultepec Castle. Ransom was succeeded by Colonel Jones M. Withers, who resigned 23 May 1848, and he was succeeded by Colonel Henry L. Webb.

The regiment served in the Mexico City campaign and was in the Battle of Contreras, Battle of Churubusco, Battle of Molino del Rey and at the Battle of Chapultepec where it took a distinguished part. At the Battle of Chapultepec it was in support of the storming force, but joined with it as a part of the assault on the citadel. Sixteen officers and eleven enlisted men of the regiment were mentioned by name in the report of Major General Gideon Pillow for meritorious conduct in this battle, among the former being Second Lieutenant R. C. Drum, later a general. Another officer who served with the regiment was Major William B. Taliaferro, who became a major general in the Confederate States Army during the American Civil War.

Following the end of the war, the regiment returned to the United States where, by 26 August 1848, its officers and soldiers were discharged and the regiment was disbanded.