The 142nd Regiment, Illinois Volunteer Infantry was an infantry regiment that served in the Union Army during the American Civil War.

It was among the scores of regiments that were raised in the summer of 1864 as Hundred Days Men, part of an effort to augment existing manpower for an all-out push to end the war within 100 days.

At Camp Butler, Illinois, two companies were added, and the regiment was mustered into Federal service on June 18, 1864, for a one-hundred-day enlistment.

The regiment was mustered out of service on October 27, 1864, at Chicago, Illinois.

The regiment suffered 30 enlisted men who died of disease, for a total of 30 fatalities.