The 21st Ohio Infantry Regiment was an infantry regiment in the Union Army during the American Civil War. Mostly an all-volunteer unit, with the exception of a few draftees, the 21st Ohio served for both ninety-day and three-year enlistments and fought exclusively in the Western Theater. It saw action in some of the war's bloodiest battles including Stones River, Chickamauga, the Atlanta Campaign, and Sherman's March to the Sea.