The 1st Regiment Massachusetts Volunteer Infantry was an infantry regiment in the Union army during the American Civil War. It was the first regiment to leave Massachusetts for a three-year term (several had previously left for 90-day terms) in response to President Abraham Lincoln's May 3, 1861, call for three-year regiments. It was also the first three-year regiment from any state to reach Washington, D.C., for federal service. The core of the regiment was five companies from the 1st Massachusetts Volunteer Militia, a peace-time unit which was formed in 1858, replacing an earlier, disbanded unit of the same designation. Five companies of new recruits were added to the regiment and the unit was mustered in by companies beginning May 23, 1861, at Camp Cameron in Cambridge, Massachusetts.