The 20th Maine Infantry Regiment was a volunteer regiment of the United States Army (Union Army) during the American Civil War (1861–1865), most famous for its defense of Little Round Top at the Battle of Gettysburg in Gettysburg, Pennsylvania, July 1–3, 1863. The 133rd Engineer Battalion of the Maine Army National Guard and the United States Army today carries on the lineage and traditions of the 20th Maine.