The 9th Massachusetts Infantry Regiment was a Union Army regiment which was active from 11 June 1861 to 21 June 1864 during the American Civil War. The regiment, financed by the publisher Patrick Donahoe and commanded by Colonel Thomas Cass, was recruited in Boston from among the city's Irish-American community. The regiment was attached to the Army of the Potomac and first served in the Peninsula Campaign and the Seven Days Battles, and it went on to fight in all of the Army of the Potomac's major battles until the Battle of Cold Harbor, having lost 15 officers and 194 enlisted men during the war.