The 21st Wisconsin Infantry Regiment was a volunteer infantry regiment that served in the Union Army during the American Civil War.

The 21st Wisconsin Infantry was established by Governor Edward Salomon as one of several new regiments to fill President Abraham Lincoln's call for 300,000 three-year volunteers.

[1] The volunteers of the 21st Wisconsin Infantry were mostly drawn from the counties of Fond du Lac, Winnebago, Outagamie, Waupaca, Calumet, and Manitowoc.

[2]: 686 Organized at Oshkosh, Wisconsin, and mustered on September 5, 1862.

Attached to 28th Brigade, 3rd Division, Army of the Ohio, September, 1862.

2nd Brigade, 1st Division, 14th Army Corps, to April, 1864.

[3] SERVICE.--Pursuit of Bragg to Crab Orchard, Ky., October 1–16, 1862.

Middle Tennessee (or Tullahoma) Campaign June 23 – July 7.

Passage of the Cumberland Mountains and Tennessee River and Chickamauga (Ga.) Campaign August 16 – September 22.

Davis Cross Roads, near Dug Gap, September 11.

Operations on line of Pumpkin Vine Creek and battles about Dallas, New Hope Church and Allatoona Hills May 25 – June 5.

Operations about Marietta and against Kenesaw Mountain June 10 – July 2.

[3] The 21st Wisconsin suffered 5 officers and 117 enlisted men killed in action or who later died of their wounds, plus another 3 officers and 180 enlisted men who died of disease, for a total of 305 fatalities.